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AMERICAN  SCIENCE   SERIES 


AN  INTRODUCTION 


TO 


GENERAL    BIOLOGY 


BY 

WILLIAM  T.   SEDGWICK,   Ph.D. 

i'ro/essor  of  Biology  in  the  Massachusetts  Institute  of  Technology^  Boston 

AND 

EDMUND  B.   WILSON,   Ph.D. 

Professor  of  Zoology  in  Columbia  College,  New  York 


SECOND  EDITION,  REVISED  AND  ENLARGED 


NEW  YORK 

HENRY  HOLT  AND  COMPANY 

1899 


Copyright,  1886, 1895, 

BY 

HENRY  HOLT  &  CO. 


PEEFACE  TO  THE  FIRST  EDITION. 


Several  years  ago  it  was  our  good  fortune  to  follow,  as  grad- 
uate students,  a  course  of  lectures  and  practical  study  in  General 
Biology  under  the  direction  of  Professor  Martin,  at  Jolins  Hop- 
kins University.  So  interesting  and  suggestive  was  the  general 
method  employed  in  this  course  which,  in  its  main  outlines,  had 
been  marked  out  by  Huxley  and  Martin  ten  years  before,  that 
we  were  persuaded  that  beginners  in  biology  should  always  be  in- 
troduced to  the  subject  in  some  similar  way.  The  present  work 
thus  owes  its  origin  to  the  influence  of  the  autliors  of  the 
' '  Elementary  Biology, ' '  our  deep  indebtedness  to  whom  we 
gratefully  acknowledge. 

It  is  still  an  open  question  whether  tlie  beginner  should  pur- 
sue the  logical  but  difficult  course  of  working  upwards  from  tlie 
simple  to  the  complex,  or  adopt  the  easier  and  more  practical 
method  of  working  downwards  from  famihar  higlier  forms. 
Every  teacher  of  the  subject  knows  how  great  are  the  practical 
difficulties  besetting  the  novice,  who,  provided  for  tlie  lirst  time 
with  a  compound  microscope,  is  confronted  with  Yeast,  Proto- 
coccus,  or  Amoeba ;  and  on  the  other  hand,  how  hard  it  is  to  sift 
out  what  is  general  and  essential  from  the  heterogeneous  details 
of  a  mammal  or  a  flowering  plant.  In  the  hope  of  lessening  the 
practical  difficulties  of  the  logical  method  we  venture  to  submit  a 
course  of  preliminary  study,  which  we  have  used  for  some  time 
with  our  own  classes,  and  have  found  practical  and  effective. 

It  has  not  been  our  ambition  to  prepare  an  exhaustive  trea- 
tise. We  have  sought  only  "to  lead  beginners  in  biology  from 
familiar  facts  to  a  better  knowledge  of  how  living  things  are 
built  and  how  they  act,  such  as  may  rightly  take  a  place  in  gen- 


^       ^fS  /■ 


111 


IV  PREFACE  TO  THE  FIRST  EDITION. 

eral  education  or  may  afford  a  basis  for  further  studies  in  General 
Biology,  Zoology,  Botany,  Physiology,  or  Medicine. 

Believing  that  biology  should  follow  the  example  of  physics 
and  chemistry  in  discussing  at  the  outset  the  fundamental  prop- 
erties of  matter  and  energy,  we  have  devoted  the  first  three 
chapters  to  an  elementary  account  of  living  matter  and  vital  en- 
ergy. In  the  chapters  which  follo^v,  these  facts  are  applied  by 
a  fairly  exhaustive  study  of  a  representative  animal  and  plant,  of 
considerable,  though  not  extreme,  complexity — a  method  which 
we  believe  affords,  in  a  given  tune,  a  better  knowledge  of  vital 
phenomena  than  can  be  acquired  by  more  superficial  study  of  a 
larger  number  of  forms.  We  are  satisfied  that  the  fern  and  the 
earthworm  are  for  this  purpose  the  best  available  organisms,  and 
that  their  study  can  be  made  fruitful  and  interesting.  The  last 
chapter  comprises  a  lirief  account  of  the  j)rinciples  and  outlines 
of  classification  as  a  guide  in  subsequent  studies. 

After  this  introductory  study  the  student  will  be  well  pre- 
pared to  take  up  the  one-celled  organisms,  and  can  pass  rapidly 
over  the  ground  covered  by  such  works  as  Huxley  and  Martin's 
''Practical  Biology,"  Brooks's  "Handbook  of  Invertebrate 
Zoology,"  Arthur,  Barnes  and  Coulter's  "Plant  Dissection,"  or 
the  second  part  of  this  book,  which  is  well  in  hand  and  will 
probably  be  ready  in  the  course  of  the  following  year. 

The  directions  for  practical  study  are  intended  as  suggestions, 
not  substitutes,  for  individual  effort.  We  have  striven  to  make 
the  work  useful  as  well  in  the  class-room  as  in  the  laboratory, 
and  to  this  end  have  introduced  many  illustrations.  The  gener- 
ositv  of  a  friend  has  enabled  us  to  enlist  the  skill  of  our  friend 
Mr.  James  H.  Emerton,  who  has  drawn  most  of  the  original 
figures  from  nature,  under  our  direction.  We  have  also  been 
greatly  aided  in  the  preparation  of  the  figures  by  Mr,  William 
Claus  of  Boston. 

September,  1886. 


PKEFACE  TO  THE  SECOND  EDITION. 


It  was  originally  our  intention  to  publish  this  work  in  two 
parts,  the  first,  which  appeared  in  1886,  being  intended  as  an 
introduction,  while  the  second  was  to  form  the  main  body  of  tlie 
work  and  to  include  the  study  of  a  series  of  type-forms.  The 
pressure  of  other  work,  however,  delayed  the  completion  of  tb.e 
second  part,  and  meanwhile  several  laboratory  manuals  appeared 
which  in  larsie  measure  obviated  the  need  of  it.  Nevertheless 
the  use  of  the  introductory  volume  by  teachers  of  Biology, 
and  its  sale,  slowly  but  steadily  increased.  It  soon  appeared, 
however,  that  in  some  cases  the  work  was  being  employed  not 
merely  as  an  introduction,  as  its  authors  intended,  but  as  a 
complete  course  in  itself;  though  the  wish  was  often  expressed 
that  the  number  of  types  were  somewhat  larger.  These  facts, 
and  the  many  obvious  defects  in  the  original  volume,  induced 
us  to  undertake  the  preparation  of  a  second  and  extended  editioiu 

"With  increased  experience  our  ideas  have  undergone  some 
change.  We  are  as  firmly  convinced  as  ever  that  General  Biol- 
ogy, as  an  introductory  subject,  is  of  the  very  first  impoi-tance ; 
but  we  are  equally  persuaded  that  it  must  not  trespass  too  far 
upon  the  special  provinces  of  Zoology  and  Botany.  The  present 
edition,  therefore,  differs  from  the  original  in  these  respects: 
first,  while  the  introduction  has  been  extended  so  as  to  in- 
clude representatives  of  the  unicellular  organisms  {Amoeha, 
Infusoria^  Protococcus^  Yeasts,  Bacteria),  the  publication  of  a 
second  volume  has  been  abandoned.  It  is  lioped  tliat  tlie  work 
as  thus  extended  may  serve  a  double  purpose,  viz.,  either  to 
be  used  as  an  introduction  to  subsequent  study  in  Zoology,  Bota- 
ny, or  Physiology;  or  as  a  complete  elementary  course  for 
general  students  to  whom  the  minutiae  of  tliese  more  special  sub- 
jects are  of  less  importance  than  the  fundamental  facts  of  vital 
structure  and  function.     We  believe  that  a  sound  knowledge  of 


\i  PREFACE  TO   THE  SECOND  EDITION. 

these  facts  can  be  conveyed  by  tlie  metliod  of  study  liere  out- 
lined ;  but  we  must  emphatically  insist  that  neither  this  nor  any 
Xother  method  will  give  good  results  unless  rightly  used,  and  that 
C  this  work  is  not  designed  to  be  a  complete  text-book.  Probably 
few  teachers  will  find  it  desirable  to  go  over  the  whole  of  the 
ground  here  laid  out,  and  we  hope  that  still  fewer  will  be  inclined 
to  confine  their  work  strictly  to  it.  Even  in  a  brief  course  the 
student  may,  after  going  over  certain  portions  of  this  work,  be 
made  acquainted  with  the  leading  types  of  plants  and  animals ; 
and  this  may  be  rapidly  accomplished  if  the  introductory  work, 
however  limited,  has  been  carefully  done.  In  extended  courses 
we  have  sometimes  found  it  desirable  to  postpone  certain  parts  of 
the  introductory  work,  returning  to  them  at  a  later  period. 

A  second  modification  consists  in  placing  the  study  of  the 
animal  before  that  of  the  plant,  which  plan  on  the  whole  appears 
desirable,  especially  for  students  who  have  not  been  well  trained 
in  other  branches  of  science.  The  main  reason  for  this  lies  in  the 
greater  ease  with  which  the  physiology  of  the  animal  can  be  ap- 
proached ;  for  there  is  no  doubt  that  beginners  find  the  nutritive 
problems  of  the  plant  abstruse  and  difiicult  to  grasp  until  a  cer- 
tain familiarity  with  vital  phenomena  has  been  attained ;  while 
most  of  the  physiological  activities  of  the  animal  can  be  readily 
illustrated  by  well-known  operations  of  the  human  body. 

The  third  change  is  the  omission  of  the  laboratory  directions, 
these"  havih^~beentound  "TmsuitabTe!  The  needs~of  different 
teachers  differ  so  widely  that  it  is  impossible  to  draw  up  a  scheme 
that  shall  answer  for  all.  In  place  of  the  laboratory  directions  for 
students  we  have  therefore  given,  in  an  appendix,  a  series  (fTprac- 
ticaFsucrffestions  to  teachers,  leaving^  it  to  them  to  work  OTxde- 
taHed  directions,  if  desirecT/  by  the  help  of  the  standard  labora- 
tory manuals.  These  suggestions  are  the  result  of  a  good  deal  of 
experience  on  the  part  of  many  teachers  besides  ourselves,  and 
we  hope  they  will  be  found  useful  in  procuring  and  preparing 
material  (often  a  matter  of  considerable  difficulty),  and  in  decid- 
ing just  what  the  student  may  reasonably  be  expected  to  do. 

For  the  rest,  the  original  matter  has  been  thoroughly  re^dsed, 
numerous  errors  have  been  corrected,  and  many  additions  made, 
particularly  on  the  physiological  side. 

September,  1895. 


TABLE  OF  CONTENTa 


CHAPTER  I, 

INTBODVCTOBY. 

Living  things  and  lifeless  things.  The  contrast  and  the  likeness  hetweea 
living  matter  and  lifeless  matter.  The  journey  of  lifeless  matter 
through  living  things.  Analogy  between  a  fountain,  a  flame  or  a 
whirlpool,  and  a  living  organism.  Living  matter  is  lifeless  matter  in 
a  peculiar  state  or  condition.  Its  characteristic  properties.  Biology, 
its  scope  and  its  subdivisions.  The  Biological  sciences.  The  relations 
of  -Biology  to  Zoology  and  Botany,  Morphology  and  Physiology. 
Definitions  and  inter-relations  of  the  biological  sciences.  Psychol- 
ogy, Sociology.    Definition  of  General  Biology.  .,...<,,„..., 1 


CHAPTER  n, 

THE  STRUCTURE  OF  LiriNG   THINGS. 

Their  occurrence  and  their  size  Organisms  composed  of  organs.  Func- 
tions. Organs  composed  of  tissues.  Differentiation.  Tissues  com- 
posed of  cells,  DefinitionSo  Unicellular  organisms.  Living  organ- 
isms contain  lifeless  matter.  Lifeless  matter  occurs  in  living 
tissues  and  cells.  Examples,  Lifeless  matter  increases  relatively 
with  age.  Summary  statement  of  the  structure  of  living  things. 
The  organism  as  a  whole — the  Body — more  important  than  any  of  its 
parts <> o 9 

CHAPTER  III. 

PROTOPLASM  AND  THE  CELL, 

Protoplasm  "the  physical  basis  of  life."  Historical  sketch.  The  com- 
pound microscope  and  the  discovery  of  cells  in  cork.  The  achromatic 
objective.  The  cell-theory  of  Schlelden  and  Schwann,  Virchow 
and  Max  Schultze.  Modern  meaning  of  the  term  "cell/*  The  dis- 
covery of  protoplasm  and  sarcode  and  of  their  essential  similarity. 

•  • 

Vll 


nii  TABLE  OF  CONTENTS. 

PAQB 

Purkinje.  Von  Mohl.  Cohn.  Schiiltze.  Appearance  and  structure 
of  protoplasm,  A  typical  cell.  Its  parts.  Cytoplasm  and  the  nucleus. 
The  origin  of  cells.  Segmentation  of  the  egg,  differentiation  of  the 
tissues,  the  genesis  of  the  "  body,"  and  the  physiological  division  of 
iabor.  Protoplasm  at  work.  Muscular  contractions.  Amo&ba  on  its 
travels.  "Rotation"  in  Nitella  and  AnacJtaris.  "Circulation"  of 
the  protoplasm  in  hair- cells  of  spiderwort.  Ciliary  motion.  The 
sources  of  protoplasmic  energy.  Metabolism  and  its  phases.  Vital 
energy  does  not  imply  a*' vital  force.'*  The  chemical  relations  of 
protoplasm:  proteids,  carbohydrates,  and  fats.  Physical  Relations: 
temperature,  moisture,  electricity,  etc.  The  protoplasm  of  plants  and 
A  animals  similar  but  not  identical c 20 


CHAPTER  IV. 
THE  BIOLOGY  OF  AN  ANIMAL:    THE  COMMON  EARTHWORM. 

4.  representative  animal.  Earthworms  taken  as  a  type.  Their  wide  dis- 
tribution. The  common  earthworm.  Its  name  ;  habitat ;  habits  ; 
food  ;  castings ;  influence  on  soils  ;  burial  of  objects ;  senses.  Its 
differentiation:  antero-posterior  and  dorso-ventral.  Its  symmetry: 
bilateral  and  serial.  Plan  of  the  earthworm's  body.  Organs  of  the 
body  and  the  details  of  their  arrangement  in  systems:  alimentary; 
circulatory;  excretory;  respiratory;  motor;  nervous;  sensitive;  etc,.    41 

CHAPTER  V. 

THE  BIOLOGY  OF  AN  ANIMAL:    THE  COMMON  EARTHWORM  {Continued). 

Definition  of  reproduction.  The  germ-cells.  Sexual  and  asexual  repro- 
duction. Regeneration.  The  reproductive  system  of  the  earthworm. 
Its  copulation  and  egg-laying.  The  process  of  fertilization,  and  the 
segmentation  or  cleavage  of  the  egg.  The  making  of  the  body.  The 
gastrula.  The  three  germ-layers  :  ectoblast,  entoblast,  mesoblast. 
Brief  statement  of  the  phenomena  of  cell-division,  and  of  nuclear 
division  or  karyokinesis.  The  making  of  the  organs.  The  fate  of 
the  germ-layers.     The  germ-plasm » . » 72 

CHAPTER  VL 

THE  BIOLOGY  OF  AN  ANIMAL:   THE  COMMON  EARTHWORM  {Continued), 

The  microscopic  anatomy  or  histology  of  the  earthworm.  The  funda- 
mental animal  tissues  and  their  constituent  cellular  elements.  Epi- 
thelial, muscular,  nervous,  germinal,  blood,  and  connective  tissues, 
and  their  distribution  in  the  various  organs.  Microscopic  structure 
of  the  body-wall ;  of  the  alimentary  canal ;  of  the  blood-vessels ;  of 
the  dissepiments  ;  of  the  nervous  system,  ganglia  ;  etc. .. 90 


TABLE  OF  CONTENTS  ix 

CHAPTER  VII. 

TEE  BIOLOGY  OF  AN  ANIMAL:    THE  COMMON  EARTHWORM  iContinued.) 

PAGS 

General  Physiology.  The  animal  and  its  environment.  Definitions. 
Adaptation,  structural  and  functional,  of  organism  to  environmento 
Origia  of  adaptations.  Elfect  of  their  persistence  and  accumulation. 
Natural  selection  through  the  survival  of  the  fittest.  The  need  of  an 
income  of  food  to  supply  matter  and  energy.  Nature  of  the  income. 
The  food  and  its  journey  through  the  body.  Alimentation.  Diges- 
tion and  absorption.  Circulation.  Metabolism.  The  outgo.  Inter- 
action of  the  animal  and  the  environment.     Summary 97 

CHAPTER  VIII. 

THE  BIOLOGY  OF  A  PLANT:    THE  COMMON  BRAKE  OR  FERN. 

A  representative  plant.  Ferns  taken  as  a  type.  Their  w^ide  distribution. 
The  common  brake.  Its  name,  habitat,  size,  etc.  General  morphol- 
ogy of  its  body.  Its  differentiation,  antero-posterior  and  dorso-ventral. 
Its  bilateral  symmetry.  The  underground  stem.  Origin  and  arrange- 
ment of  the  leaves.  Internal  structure  of  the  rhizome  and  the  three 
great  tissue-systems.  The  elementary  tissues  of  plants.  Histology  of 
the  rhizome.  Roots  and  branches.  Embryonic  tissue  and  the  apical 
cell.  How  the  rhizome  grows.  The  frond  or  leaf  of  Pieris  and  its 
structure.     Chlorophyll-bodies.     Stomata.     Veins 105 

CHAPTER  IX. 

THE  BIOLOGY  OF  A  PLANT:    THE  COMMON  BRAKE  (Continued). 

The  various  methods  of  reproduction  in  Pteris.  Sporophore'  and 
oophore.  Alternation  of  generations.  Sporangia.  Spores.  Ger- 
mination of  the  spores.  Protonema.  Prothallium.  The  sexual 
organs.  Antheridia.  Male  germ-cells.  Archegonia.  Female  germ- 
cells.  Fertilization.  Segmentation.  Differentiation  of  the  tissues. 
The  making  of  the  body » 130 

CHAPTER  X. 

THE  BIOLOGY  OF  A  PLANT:   THE  COMMON  BRAKE  (Continued). 

Physiology.  The  fern  and  its  environment.  Its  adaptation.  A  defini- 
tion of  life.  The  need  of  an  income  of  matter  and  energy.  Income 
of  Pteris.  Its  power  of  making  foods,  especially  starch.  The  circu- 
lation of  foods  through  the  plant-body.  Metabolism.  Outgo.  Res- 
piration.     Interaction  of  the  fern  and   the  environment.      Special 


TABLE  OF  CONTENTS. 

PAGB 

physiology  of  the  tissue-systems  and  of  reproduction.  The  question 
of  old  age.  A  comparison  of  the  fern  with  the  earthworm,  and  of 
plants  in  general  with  animals  in  general.  The  physiological  im- 
portance of  the  chlorophylless  plants 144 


CHAPTER  XI. 

THE  UNICELLULAR  ORGANISMS. 

The  multicellular  body.  Its  origin  in  continued,  but  incomplete,  cell- 
division.  The  unicellular  body.  Its  origin  traced  to  complete  cell- 
division.  The  multicellular  body  and  the  unicellular  body  as 
individuals.  Unicellular  forms  physiologically  "  organisms."  Special 
importance  of  their  structural  simplicity.  "Organisms  reduced  to 
their  lowest  terms. " 156 


CHAPTER  XII. 

UNICELLULAR  ANIMALS. 
A.  Amceba. 

General  Account.  Habitat.  Form.  The  "  Proteus  animalcule.'*  Ap- 
pearance. Pseudopodia.  Locomotion.  Foods.  The  encysted  state. 
Structure  of  the  unicellular  body.  Cytoplasm.  Nucleus.  Vacuoles. 
Reproduction  by  fission.  Physiology.  The  fundamental  physiological 
properties  of  protoplasm  as  displayed  in  Aynoeba.  The  (question  of 
■old  age.  Related  forms.  The  Rhizopoda  or  pseudopodial  Protozoa. 
Arcella.  Difflugia.  The  "sun-animalcule."  The  Foramenifera. 
The  Radiolaria , 158 


CHAPTER  XIII. 

UNICELLULAR  ANIMALS  {Continued). 

B.  Infusoria. 

General  account.  Habitat.  The  "slipper-animalcule."  The  "bell- 
animalcule."  Paramcecium.  Its  form,  structure,  and  habits.  Cyto- 
plasm; trichocysts;  vacuoles;  nuclei;  mouth;  oesophagus;  anal  spot. 
The  encysted  state.  Reproduction  by  agamogenesis;  by  conjugation; 
amphimixis.  Voriicella.  Its  form,  structure,  etc.  Its  reproduction 
by  fission,  endogenous  division,  and  conjugation.  Microgamete  and 
macrogamete.  Related  forms=  Euglena;  Zoothamnion;  CarcJiesium; 
Epistylis;  etc.  Physiology  of  the  Infusoria.  Herbivorous,  carniv- 
orous, and  omnivorous  infusoria.  Analogy  with  higher  forms.  The 
problem  of  chlorophyll  in  animals.  S3^mbiosis.  Vegetating  animals. 
The  claim  of  unicellular  animals  to  be  regarded  as  unicellular  "or- 
ganisms"; organs  in  the  cell;  etc o 168 


TABLE  OF  CONTENTS.  XI 

CHAPTER  XIV. 

UNICELLULAR  PLANTS. 
A.  Pbotococcus. 

PAGB 

General  account.  Habitat.  Morphology.  Structure.  Motile  and  non- 
motile  states.  Reproduction  by  fission.  Cell-aggregates.  Physi- 
ology. Income  and  outgo.  The  making  of  starch  from  inorganic 
matters.  The  fundamental  physiological  properties  of  protoplasm  as 
displayed  by  plants.  Comparison  of  Proiococcus  with  Aniosba,  and 
chlorophyll-bearing  plants  in  general  with  animals  in  general.  Other 
unicellular  chlorophyll-bearing  plants:  diatoms;  desmids;  Ghroocoo 
cus;  Glceocapsa;  etc 178 

CHAPTER  XV. 

UNICELLULAR  PLANTS  {Continued). 
B.  Yeast. 

General  account.  Wild  yeast  and  domesticated  yeast.  Microscopical 
examination  of  a  yeast-cake.  Morphology  of  the  yeast  cell.  Cyto- 
plasm and  nucleus.  Reproduction  by  budding  and  by  spores.  Physi- 
ology. Yeast  and  the  environment.  Dried  yeast.  Income.  Meta- 
bolism. Oiitgo.  The  minimal  nutrients  of  yeast  compared  with 
those  of  Proiococcus  and  Amoeba.  Why  yeast  is  regarded  as  a  plant. 
Top  yeast  Bottom  yeast.  Wild  yeasts.  Red  yeast.  Fermentation 
and  ferments.  Unicellular  plants  not  necessarily  at  the  bottom  of 
the  scale  of  life;  etc    184 

CHAPTER  XVI. 

UNICELLULAR  PLANTS  (Continued). 
C.  Bacteria. 

The  smallest,  most  numerous,  and  most  ubiquitous  of  known  living 
things.  Their  abundance  in  earth,  air,  milk,  water,  etc.  Comparison 
of  their  work  in  soils  with  that  of  earthworms.  Parasitic  and  sapro- 
phytic bacteria.  Their  botanical  position.  Sanitary  and  economic 
importance.  Morphology.  Structure.  Cytoplasm  and  nucleus. 
Cilia.  Their  size.  Swarming  and  the  resting  stages.  Reproduction. 
Endospores.  Arthrospores.  Physiology.  Income.  Metabolism. 
Outgo.  Ferments.  Fermentation,  Putrefaction.  Disease.  One 
species  capable  of  living  upon  inorganic  matter.  Related  forms. 
Why  bacteria  are  regarded  as  plants.  The  relations  of  bacteria  to 
temperature,  moisture,  poisons,  etc.  Sterilization,  Pasteurizing, 
disinfection,  filtration,  etc , 192 


XU  TABLE  OF  CONTENTS. 

CHAPTER  XVII 

A  HAY  INFUSION. 

PAGB 

General  account.  Results  of  microscopical  examination.  Turbidity. 
Odor.  Color.  Coustituents.  The  scene  of  important  pliysical, 
chemical,  and  biological  phenomena.  Previous  history  of  the  hay 
and  the  water.  Efiect  of  bringiug  them  together.  Causes  of  tur- 
bidity, color,  odor,  etc.  Aerobic  and  anaerobic  bacteria  thrive. 
Infusoria  multiply  and  devour  them.  Carnivorous  mfusoria  attack 
the  herbivorous.  The  struggle  for  existence.  Hay  a  green  plant 
and  the  source  of  food.  Quiet  finally  supervenes.  How  nutritive 
equilibrium  may  be  preserved  or  disturbed.  The  hay-infusion  an 
epitome  of  the  living  world 201 

APPENDIX. 

SUGGESTIONS  FOR  LABORATORY  STUDIES  AND  DEMONSTRATIONS. 

Books  for  the  laboratoiy.     Time  required  for  General  Biology  . , 205 

Special  suggestions  for  laboratory  work,  etc.,  upon  the  subjects  treated 
in  the  several  chapters  as  outlined  above,  viz.: 

Chapter  I.  Introductory  205 

II.  Structures  of  Living  Organisms , 206 

III.  Protoplasm  and  the  Cell. o. 207 

IV. -VIII.  The  Earthworm ,..  2)0 

IX.-XI.  The  Fern , ,....  213 

XII.  Amoeba 216 

XIII.  Infusoria 217 

XIV.  Protococcus 220 

XV.  Yeast o =  ..o 221 

XVI.  Bacteria 222 

XVII.  AHay  Infusion 223 

Instruments  and  Utensils o o  220 

Reagents  and  Technical  Methods c « . .  221 

Index. 227 


GENERAL  BIOLOGY. 


CHAPTER  I. 

INTRODUCTORY. 


"We  know  from  common  experience  that  all  material  things 
are  either  dead  or  alive,  or,  more  accurately,  that  all  matter  is 
either  lifeless  or  living ;  and  so  far  as  we  know,  life  exists  only 
as  a  manifestation  of  living  matter.  Living  matter  and  lifeless 
matter  are  everywhere  totally  distinct,  though  often  closely  as- 
sociated. The  most  careful  studies  have  on  the  whole  rendered 
the  distinction  more  clear  and  striking,  and  have  demonstrated 
that  living  matter  never  arises  spontaneously  from  lifeless  matter, 
but  only  through  the  immediate  influence  of  living  matter  already 
existing.  And  so,  whatever  may  have  been  the  case  at  an  earlier 
period  of  the  earth's  history,  we  are  justified  in  regarding  the 
present  line  between  living  and  lifeless  as  one  of  the  most 
clearly  defined  and  important  of  natural  boundaries. 

The  Contrast  between  Living  Matter  and  Lifeless  Matter  is  made 
the  ground  for  a  division  of  the  natural  sciences  into  two  great 
groups,  viz.  :  the  Biological  Sciences  and  the  Physical  Sciences, 
dealing  respectively  with  living  matter  and  lifeless  matter.  The 
biological  sciences  (p.  7)  are  known  collectively  as  Biology 
{/3io5,  life;  Xoyos,  a  discourse),  which  is  therefore  often  de- 
fined as  the  science  of  life,  or  of  living  things,  or  of  living  mat- 
ter.  But  living  matter,  so  far  as  we  know,  is  only  ordinary 
matter  which  has  entered   into  a  peculiar  state  or  condition. 


..A  A 


2  INTRODUCTORY. 

And  hence  biology  is  more  precisely  defined  as  tJie  science  ichich 
treats  of  viciUer  in  the  living  state. 

The  Relationship  between  Living  and  Lifeless  Matter.  Al- 
though living  matter  and  lifeless  matter  present  this  remarkable 
contrast  to  one  another,  they  are  most  intimately  related,  as  a 
moment's  reflection  will  show.  The  living  substance  of  the  human 
body,  or  of  any  animal  or  plant,  is  only  the  transformed  lifeless 
matter  of  the  food  which  has  been  taken  into  the  body  and  has 
there  assumed,  for  a  time,  the  living  state.  Lifeless  matter  in 
the  shape  of  food  is  continually  streaming  into  all  living  things 
on  the  one  hand  and  passing  out  again  as  waste  on  the  other. 
In  its  journey  through  the  organism  some  of  this  matter  enters 
into  the  living  state  and  lingers  for  a  tune  as  part  of  the  body 
substance.  But  sooner  or  later  it  dies,  and  is  then  for  the  most 
part  cast  out  of  the  body  (though  a  part  may  be  retained  within 
it,  either  as  an  accumulation  of  waste  material,  or  to  serve  some 
•aseful  purpose).  Matter  may  thus  pass  from  the  lifeless  into  the 
living  state  and  back  again  to  the  lifeless,  over  and  over  in  never- 
ending  cycles.  A  living  plant  or  animal  is  like  a  fountain  or  a 
flame  into  which,  and  out  of  which,  matter  is  constantly  stream- 
ing, while  the  fountain  or  the  flame  maintains  its  characteristic 
form  and  individuality.  It  is  "  nothing  but  the  constant  form  of 
a  similar  turmoil  of  material  molecules,  which  are  constantly 
flowing  into  the  organism  on  the  one  side  and  streaming  out  on 
the  other.  .  .  .  It  is  a  sort  of  focus  to  which  certain  material  par- 
ticles converge,  in  which  they  move  for  a  time,  and  from  which 
they  are  afterward  expelled  in  new  combinations.  The  parallel 
between  a  whirlpool  in  a  stream  and  a  living  being,  which  has 
often  been  drawn,  is  as  just  as  it  is  striking.  The  whirlpool  is 
permanent,  but  the  particles  of  water  which  constitute  it  are  in- 
cessantly changing.  Those  which  enter  it  on  the  one  side  are 
w^hirled  around  and  temporarily  constitute  a  part  of  its  mdi- 
viduality ;  and  as  they  leave  it  on  the  other  side,  their  jDlaces  are 
made  good  by  newcomers."      (Huxley.) 

How  then  is  living  matter  different  from  lifeless  matter  ? 
The  question  cannot  be  fully  answered  by  chemical  analysis,  for 
the  reason  that  this  process  necessarily  kills  living  matter,  and 
the  results  therefore  teach  us  little  of  the  chemical  conditions  ex- 
isting in  the  matter  when  alive.     Analyses,  nevertheless,  bring 


LIVING  MATTER.  S 

to  light  several  liiglilj  important  facts.  It  is  likely  that  livino- 
matter  is  a  tolerably  detinite  compound  of  a  nnmljer  of  the 
chemical  elements,  and  it  is  probably  too  low  an  estimate  to  say 
that  at  least  six  elements  must  unite  in  order  that  life  may  ex- 
ist. Moreover,  only  a  very  few  out  of  all  the  elements  are  able, 
under  any  circumstances,  to  form  this  living  partnership. 

The  most  significant  fact,  however,  is  that  there  is  no  loss  of 
weight  when  living  matter  is  killed.  The  total  weight  of  the 
lifeless  products  is  exactly  equal  to  the  weight  of  t\iQ  living  sub- 
stance analyzed,  and  if  anything  has  escaped  at  death  it  is  im- 
ponderable, and,  havhig  no  weight,  is  not  material.  It  follows- 
that  living  matter  contains  no  material  substance  peculiar  to  it- 
self, and  that  every  element  found  in  living  matter  may  be  found 
also,  under  other  circumstances,  in  lifeless  matter. 

Considerations  like  these  lead  ns  to  recognize  a  fundamental 
fact,  namely,  that  the  terms  living  and  lifeless  designate  two 
different  states  or  conditions  of  matter.  "We  do  not  know,  at 
present,  what  causes  this  difference  of  condition.  But  so  far  as. 
the  evidence  shows,  the  li^ang  state  is  never  assumed  except 
under  the  influence  of  antecedent  living  matter,  wliicli,  so  to 
speak,  infects  lifeless  matter  and  in  some  way  causes  it  to  as- 
sume the  living  state. 

Distinctive  Properties  of  Living  Matter.  Those  properties  of 
living  matter  which,  taken  together,  distinguish  it  absolutely 
from  every  form  of  lifeless  matter,  are : 

1.  Its  chemical  composition. 

2.  Its  power  of  waste  and  rejDair,  and  of  growth. 

3.  Its  power  of  reproduction. 

Living  matter  invariably  contains  substances  known  as  pro- 
teids,  which  are  believed  to  constitute  its  essential  material  basis 
(see  p.  33).  Proteids  are  complex  compounds  of  Carbon,  Oxy- 
gen, Hydrogen,  Xitrogen,  Sulphur,  and  (in  some  cases  at  any 
rate)  Phosphorus. 

It  has  been  frequently  pointed  out  that  each  of  these  six  elements  is 
remarkable  in  some  way  :  oxygen,  for  its  vigorous  combining  powers  ; 
nitrogen,  for  its  chemical  inertia ;  hydrogen,  for  its  great  molecular 
mobility  ;  carbon,  sulphur,  and  phosphorus,  for  their  allotropic  properties, 
etc.  All  of  these  peculiarities  may  be  shown  to  be  of  significance  when 
considered  as  attributes  of  living  matter.  (See  Herbert  Spencer,  Principles 
of  Biology,  vol.  i.) 


4  INTRODUCTORY. 

It  is  not,  however,  tlie  mere  presence  of  proteids  which  is 
characteristic  of  Hving  matter.  White-of -egg  (albumen)  contains 
an  abundance  of  a  typical  proteid  and  jet  is  absolutely  lifeless. 
Living  matter  does  not  simply  contain  proteids,  but  has  tbe 
jpower  to  Tnamtfacticre  tliem  out  of  other  substances ;  and  this  is 
a  property  of  li^dng  matter  exclusively. 

The  waste  and  repair  of  living  matter  are  equally  character- 
istic. The  living  substance  continually  wastes  away  by  a  kind 
of  internal  combustion,  but  continually  repairs  the  waste.  More- 
over, the  growth  of  living  things  is  of  a  characteristic  kind,  dif- 
fering absolutely  from  the  so-called  growth  of  lifeless  things. 
Crystals  and  other  lifeless  bodies  grow,  if  at  all,  by  accretion^  or 
the  addition  of  new  j)articles  to  the  outside.  Living  matter 
grows  from  within  by  intussusception.,  or  the  taking-in  of  new 
particles,  and  fitting  them  into  the  interstices  between  those 
already  present,  throughout  the  whole  mass.  And,  lastly,  liv- 
ing matter  not  only  thus  repairs  its  own  waste,  but  also  gives 
rise  by  reproduction  to  new  masses  of  living  matter  which, 
becoming  detached  from  the  parent  mass,  enter  forthwith  upon 
an  independent  existence. 

We  may  perceive  how  extraordinary  these  properties  are  by 
supposing  a  locomotive  engine  to  possess  like  powers :  to  carry 
on  a  process  of  seK-repair  in  order  to  compensate  for  wear ;  to 
grow  and  increase  in  size,  detaching  from  itself  at  intervals 
pieces  of  brass  or  iron  endowed  with  the  power  of  grooving  up 
step  by  step  into  other  locomotives  capable  of  running  them- 
selves, and  of  reproducing  new  locomotives  in  their  turn.  Pre- 
cisely these  things  are  done  by  every  li^dng  thing,  and  nothing 
like  them  takes  place  in  the  lifeless  world. 

Huxley  has  given  the  best  statement  extant  of  the  distinctive  properties 
of  living  matter,  as  follows  : 

"1.  Its  chemical  composition — containing,  as  it  invariably  does,  one 
or  more  forms  of  a  complex  compound  of  carbon,  hydrogen,  oxygen,  and 
nitrogen,  the  so-called  protein  (which  has^  never  yet  been  obtained  except 
as  a  product  of  living  bodies),  united  with  a  large  proportion  of  water, 
and  forming  the  chief  constituent  of  a  substance  which,  in  its  primary 
unmodified  state,  is  known  as  protoplasm. 

"  2.  Its  universal  disintegration  and  waste  by  oxidation^  and  its  con- 
comitant reintegration  by  the  intussuscejytion  of  new  matter.  A  process 
of  waste  resulting  from  the  decomposition  of  the  molecules  of  the  proto- 


LIVING  MATTER.  5 

plasm  in  virtue  of  which  they  break  np  into  more  highly  oxidated  products, 
which  cease  to  form  any  part  of  the  living  body,  is  a  constant  concomitant 
of  life.  There  is  reason  to  believe  that  carbonic  acid  is  always  one  of  these 
waste  products,  while  the  others  contain  the  remainder  of  the  carbon,  the 
nitrogen,  the  hydrogen,  and  the  other  elements  which  may  enter  into  the 
composition  of  the  protoplasm. 

"  The  new  matter  taken  in  to  make  good  this  constant  loss  is  either  a 
ready-formed  protoplasmic  material,  supplied  by  some  other  living  being, 
or  it  consists  of  the  elements  of  protoplasm,  united  together  in  simpler 
combinations,  which  constantly  have  to  be  built  up  into  protoplasm  by  the 
agency  of  the  living  matter  itself.  In  either  case,  the  addition  of  molecules 
to  those  which  already  existed  takes  place,  not  at  the  surface  of  the  living 
mass,  but  by  interposition  between  the  existing  molecules  of  the  latter.  If 
the  processes  of  disintegration  and  of  reconstruction  which  characterize 
life  balance  one  another,  the  size  of  the  mass  of  living  matter  remains  sta- 
tionary, while  if  the  reconstructive  process  is  the  more  rapid,  the  living 
body  grows.  But  the  increase  of  size  which  constitutes  growth  is  the 
result  of  a  process  of  molecular  intussusception,  and  therefore  differs  alto- 
gether from  the  process  of  growth  by  accretion,  which  may  be  observed  in 
crystals,  and  is  elfected  purely  by  the  external  addition  of  new  matter  ;  so 
that,  in  the  well-known  aphorism  of  Linnaeus,  the  word  '  grow '  as  applied 
to  stones  signifies  a  totally  different  process  from  what  is  called  '  growth  * 
in  plants  and  animals. 

"  3.  Its  tendency  to  undergo  cyclical  changes.  In  the  ordinary  course 
of  nature,  all  living  matter  proceeds  from  pre-existing  living  matter,  a 
portion  of  the  latter  being  detached  and  acquiring  an  independent  exist- 
ence. The  new  form  takes  on  the  characters  of  that  from  which  it  arose ; 
exhibits  the  same  power  of  propagating  itself  by  means  cf  an  offshoot ; 
and,  sooner  or  later,  like  its  predecessor,  ceases  to  live,  and  is  resolved 
into  more  highly  oxidated  compounds  of  its  elements. 

"  Thus  an  individual  living  body  is  not  only  constantly  changing  its 
substance,  but  its  size  and  form  are  undergoing  continual  modifications, 
the  end  of  which  is  the  death  and  decay  of  that  individual ;  the  continua- 
tion of  the  kind  being  secured  by  the  detachment  of  portions  which  tend 
to  run  through  the  same  cycle  of  forms  as  the  parent.  No  forms  of  matter 
which  are  either  not  living  or  have  not  been  derived  from  living  matter 
exhibit  these  three  properties,  nor  any  approach  to  the  remarkable  phe- 
nomena defined  under  the  second  and  third  heads."  {Encyclopcedia  Bri^ 
tan7iica,  9th  ed.,  art.  "  Biology,"  vol.  iii.  p.  679.) 

For  the  purposes  of  biological  study  life  must  be  regarded  as 
a  property  of  a  certain  kind  of  compounded  matter.  But  we 
are  forced  to  regard  the  properties  of  compounds  as  the  result- 
ants of  the  properties  of  their  constituent  elements,  even  though 
we  cannot  well  imagine  how  such  a  relation  exists ;  and  so  in  the 


6  INTRODVCTORY. 

long-run  we  have  to  fall  back  upon  tlie  properties  of  carbon, 
lijdi-ogen,  nitrogen,  oxygen,  etc.,  for  the  properties  of  living 
matter. 

Scope  of  Biology.  The  Biological  Sciences.  It  follows  from 
the  broad  definition  given  to  Biology  that  this  science  includes 
the  study  of  whatever  pertains  to  living  matter  or  to  living 
things.  It  considers  the  forms,  structures,  and  functions  of  living 
things  in  health  and  m  disease ;  their  habits,  actions,  modes  of 
nutrition ;  their  surroundings  and  distribution  m  space  and  time, 
their  relations  to  the  lifeless  world  and  to  one  another,  their 
sensations,  mental  processes,  and  social  relations,  their  origin  and 
then-  fate,  and  many  other  toj^ics.  It  includes  both  zoology  and 
botany,  and  deals  with  the  phenomena  of  animal  and  vegetal  life 
not  only  separately,  but  in  their  relations  to  one  another.  It 
includes  the  medical  sciences  and  vegetal  j^athology. 

The  field  covered  by  biology  as  thus  understood  is  so  wide  as 
to  necessitate  a  subdivision  of  the  subject  into  a  number  of  principal 
branches  which  are  usually  assigned  the  rank  of  distinct  sciences. 
These  are  arranged  in  a  tabular  view  on  p.  7.  The  table  shows 
two  dift'erent  ways  of  regarding  the  main  subject,  according  as 
the  table  is  read  from  left  to  right  m*  vice  versa.  Under  the  more 
usual  arrangement  biology  is  primarily  divided  into  zoology  and 
botany,  according  as  animals  or  plants,  respectively,  form  the 
subject  of  study.  Such  a  division  has  the  great  advantage  of 
practical  convenience  since,  as  a  matter  of  fact,  most  biologists 
devote  their  attention  mainly  either  to  plants  alone  or  to  animals 
alone.  From  a  scientific  j)oint  of  view,  however,  a  better  sub- 
division is  into  Morphology  {jxofi(pii^  formj  Xoyo?,  a  discourse) 
and  Physiology  (0U(Tz?,  nature^  Xoyos,  a  discourse).  The 
formel'  is  based  upon  the  facts  of  form,  structure,  and  arrange- 
ment, and  is  essentially  statical ;  the  latter  upon  those  of  action 
or  function,  and  is  essentially  dynamical.  But  morphology  and 
physiology  are  so  intimately  related  that  it  is  impossible  to  sepa- 
rate either  subject  absolutely  from  the  other. 

Besides  the  sub-sciences  given  ui  the  table  a  distinct  branch 
called  Etiology  is  often  recognized,  having  for  its  object  the  in- 
vestigation of  the  causes  of  biological  23lienomena.  But  the  sci- 
entific study  of  every  phenomenon  has  for  its  ultimate  object  the 
discovery  of  its  cause.     -Etiology  is  therefore  inseparable  from 


THE  BIOLOGICAL  SCIENCES. 


'  Morphology. 

The  science 

of  form, 

structure, 

etc. 

Essentially 

statical. 


Biology. 

The 
science  of 
all  living 

things ; 

i.e.,  of 
matter  in 
the  living 

state. 


Physiology. 

The  science 
of  action  or 
function. 
Essentially 
dynamical. 


Aiiatotny. 
The  science  of  struc- 
ture ;  the  term  being 
usually  applied  to  the 
coarser  and  more  ob- 
vious composition  of 
plants  or  animals. 

Histology. 

Microscopic  anatomy. 
The  ultimate  optical 
analysis  of  structure 
by  the  aid  of  the 
m  i  c  r  o  s  c  op  e  ;  sepa- 
rated from  anatomy 
only  as  a  matter  of 
convenience. 

Taxonomy  or  Classifi- 
€atio)t. 

The  classification  of 
living  things.  Based 
chiefly  on  phenomena 
of  structure. 

Distribution. 

Considers  the  position 
of  living  things  in 
space  and  time,  their 
distribution  over  the 
present  face  of  the 
earth  and  their  distri- 
bution and  succession 
at  former  periods,  as 
displayed  in  fossil  re- 
mains. 

Embryology. 

The  science  of  develop- 
ment from  the  germ. 
Includes  many  mixed 
problems  pertaining 
both  to  morphology 
and  physiology.  At 
present  largely  mor- 
phological. 

Physiology. 

The  special  science  of 
the  functions  of  the 
individual  in  health 
and  in  disease  ;  hence 
including  Pathology. 

Psychology. 

The  science  of  mental 
phenomena. 

Sociology. 

The  science  of  social 
life,  i.e.,  the  life  of 
communities,  wheth- 
er of  men  or  of  lower 
animals. 


Botany,      y 

The  science 
of  vegetal 

living 

matter  or 

plants. 


\  Biology. 

The 

science  of 

all  living 

things ; 

i.e.,  of 

matter  in 

the  living 

state. 


1-     Zoology. 

The  science 
of  animal 

living 
matter  or 
animals. 


8  INTROD  UCTOR  T. 

any  of  the  .several  branches  of  biology  and  need  not  be  assigned 
an  independent  place. 

Psychology  and  Sociology  are  not  yet  generally  admitted  to 
constitute  branches  of  biology,  and  it  is  customary  and  con- 
venient to  set  them  apart  from  it.  The  establishment  of  the 
theory  of  evolution  has  clearly  shown,  however,  that  the  study 
of  these  sciences  is  inseparable  from  that  of  biology  in  the  ordi- 
nary sense.  The  instincts  and  other  mental  actions  of  the  lower 
animals  are  as  truly  subjects  of  ^psychological  as  of  physiological 
inquiry ;  the  complex  social  life  of  such  animal  communities  as 
we  hnd,  for  instance,  among  the  bees  and  ants  are  no  less  truly 
problems  of  Sociology. 

It  will  be  observed  that  in  the  scheme  morphology  and  physi- 
ology overlap ;  that  is,  there  are  certain  biological  sciences  in 
which  the  study  of  structure  and  of  action  cannot  be  sej^arated. 
This  is  especially  true  of  embryology,  which  considers  the  suc- 
cessive stages  of  embryonic  structure  and  also  the  modes  of 
action  by  which  they  are  produced.  And  Una! ly  it  must  not  be 
forgotten  that  any  particular  arrangement  of  the  biological  sci- 
ences must  be  in  the  main  a  matter  of  convenience  only ;  for  it 
is  impossible  to  study  any  one  order  of  phenomena  in  complete 
isolation  from  all  others. 

The  term  General  Biology  does  not  designaie  a  particular 
member  of  the  group  of  biological  sciences,  but  is  only  a  con- 
venient phrase,  which  has  come  into  use  for  the  general  introduc- 
tory study  of  biology.  It  bears  precisely  the  same  relation  to 
biology  that  general  chemistry  bears  to  chemistry  or  general 
physics  bears  to  physics.  It  includes  an  examination  of  the  gen- 
eral properties  of  living  matter  as  revealed  in  the  structures  and 
actions  of  particular  living  things,  and  may  sei*ve  as  a  basis  for 
subsequent  study  of  more  special  branches  of  the  science.  It 
deals  with  the  broad  characteristic  phenomena  and  laws  of  life  as 
illustrated  by  the  thorough  comparative  study  of  a  series  of 
plants  and  animals  taken  as  representative  types;  but  in  this 
study  the  student  should  never  lose  sight  of  the  fact  that  all  the 
varied  phenomena  which  may  come  under  his  observation  are  in 
the  last  analysis  due  to  the  properties  of  matter  in  the  living 
state^  and  that  this  matter  and  these  properties  are  the  real  goal 
of  the  study. 


CHAPTEE  II. 


THE  STRUCTURE   OF  LIVING  THINGS.    ORGANISMS. 


Lifeless  things  occur  in  masses  of  the  most  various  sizes 
and  forms,  and  may  differ  widely  in  structure  and  chemical  com- 
position. Living  things,  on  the  other  hand,  occur  only  in  rela- 
tively small  masses,  of  which  perhaps  the  largest  are,  among 
plants,  the  great  trees  of  California  and,  among  animals,  the 
whales ;  while  the  smallest  are  the  micro-organisms  or  bacteria. 
Moreover,  the  individual  masses  in  which  living  things  occur 
possess  a  peculiar  and  characteristic  structure  and  chemical  com- 
position which  have  caused  them  to  be  known  as  organisms^  and 
their  substance   as    organic.     All  organisms   are  built  up  to  a 

remarkable  extent  in  the  same  way  and  of  the  same  materials, 


Fig,  1.  (After  Sachs.)— Longitudinal  section  through  the  growing  apex  of  a  young 
pine-shoot.  The  dotted  portion  represents  the  protoplasm,  the  narrow  lines  be^ 
ing  the  partition-walls  composed  of  cellulose  (CsHjoOs).     (Highly  magnified.) 

and  we  may  conveniently  begin  a  study  of  living  things  with  the 
larger  and  more  complex  forms,  which  exhibit  most  clearly 
those  structural  peculiarities  to  which  we  have  referred. 

Organisms  composed  of  Organs.  Functions.  It  is  character- 
istic of  any  living  body — for  example,  a  rabbit  or  a  geranium — 
that  it  is  composed  of  unlike  parts,  having  a  structure  whicli 
enables  them  to  perform  various  operations  essential  or  accessory 
to  the  life  of  the  w^hole.      The  plant  has  stem,  roots,  branches, 

leaves,   stamens,   pistil,  seeds,   etc.  ;  the  animal  has  externally 

9 


10 


THE  STRUCTURE  OF  LIVING   THINGS. 


head,  trunk,  limbs,  eyes,  ears,  etc.,  and  internally  stomach,  in- 
testines, liver,   lungs,  heart,   brain,    and   many  other   parts  of 


Fig.  2.— Cross-section  through  part  of  the  young  leaf  of  a  fern  (Pteris  cujuilina)^ 
showing  thick- walled  cells ;  most  of  the  walls  are  double.  The  granular  sub- 
stance is  protoplasm.  Most  of  the  cells  contain  a  large  central  cavity  (vacuole) 
filled  with  sap,  the  protoplasm  having  been  reduced  to  a  thin  layer  inside  the 
partitions.  Nuclei  are  shown  in  some  of  the  cells,  and  lifeless  grains  of  starch 
in  others :  ?i,  nuclei ;  s,  starch ;  r,  vacuole ;  u\  double  partition-wall.    (  X  500.) 

the  most  diverse  structure.  These  parts  are  known  as  oi^ganSy 
and  the  living  body,  because  it  possesses  them,  is  called  an  or- 
ganism. 

The  word  organism,  as  here  used,  applies  best  to  the  higher  animals 
and  plants.  It  will  be  seen  in  the  sequel  that  there  are  forms  of  life  so 
simple  that  organs  as  here  defined  can  scarcely  be  distinguished.  Such 
living  things  are  nevertheless  really  organisms  because  they  possess 
parts  analogous  in  function  to  the  well-defined  organs  of  higher  form. 
(See  p.  157.) 

Since  organisms  are  composed  of  unlike  parts,  they  are  said 
to  be  heterogeneous  in  structure.  They  are  also  heterogeneous 
in  action,  the  different  organs  performing  different  operations 
Q,2XiQA  functions.  For  instance,  it  is  the  function  of  the  stomach 
to  digest  food,  of  the  heart  to  pump  the  blood  into  the  vessels, 
of  the  kidneys  to  excrete  waste  matters  from  the  blood,  and 
of  the  brain  to  direct  the  functions  of  other  organs.  A  similar 
diversity  of   functions   exists   in  plants.      The  roots  hold  the 


ORGANS  AND   TISSUES. 


H 


Fig.  3.  (After  Sachs.)— Cross-section 
through  a  group  of  dead,  thick- 
walled  wood-cells  from  the  stem  of 
maize.  The  cells  contain  only  air  or 
water.    (Highly  magnified.) 


plant  fast  and  absorb  various  substances  from  tlie  soil ;  the  stem 
supports  the  leaves  and  flowers 
and  conducts  the  sap ;  the  leaves 
absorb  and  elaborate  portions  of 
the  food;  and  the  reproductive 
organs  of  the  flower  serve  to 
form  and  bring  to  maturity  seeds 
destined  to  give  rise  to  a  new  gen- 
eration. 

Heterogeneity  of  the  kind 
just  indicated,  accompanied  by  a 
division  of  labor  among  the 
parts,  is  one  of  the  most  char- 
acteristic features  of  living  things, 
and  is  not  known  in  any  mass  of 
lifeless  matter,  however  large  and 
complex. 

Organs  composed  of  Tissues.  Differentiation.  In  the  next 
place,  it  is  to  be  observed  that  the  organs  also,  when  fully 
formed,  are  not  homogeneous,  but  are  in  turn  made  up  of 
different  parts.  The  human  hand  is  an  organ  w^hich  consists 
of  many  parts,  differing  widely  in  structure  and  function.  On 
the  outside  are  the  skin,  the  hairs,  the  nails ;  inside  are  bones, 
muscles,  tendons,  ligaments,  blood-vessels,  and  nerves.  The  leaf 
of  a  plant  is  an  organ  consisting  of  a  woody  framework  (the 
''  veins  I')  which  supports  a  green  pulp,  the  whole  being  covered 
on  the  outside  by  a  delicate  transparent  skin.  In  like  manner 
every  organ  of  the  higher  plants  or  animals  may  be  resolved 
into  different  parts,  and  these  are  known  as  tissues.  The 
tissues  of  fully  formed  organs  are  often  very  different  from  one 
another,  as  in  tlie  cases  just  mentioned ;  that  is,  they  are  well 
differentiated;  but  frequently  in  adult  organs,  and  always  in  those 
which  are  sufficiently  young,  the  tissues  shade  gradually  into 
one  another,  so  that  no  definite  line  can  be  drawn  between  them. 
In  such  cases  they  are  said  to  be  less  differentiated.  For  ex- 
ample, in  the  full-grown  leaf  of  a  plant  the  woody  framework,  the 
green  cells,  and  the  skin  exist  as  three  plainly  different  tissues. 
But  in  younger  leaves  these  same  tissues  are  less  different,  and 
in  very  young  leaves,  still  in  the  bud,  there  are  no  visible  differ- 


12 


THE  STRUCTURE  OF  LIVING   THINGS. 


ences  and  the  whole  organ  is  very  nearly  homogeneous.  In  this 
case  the  tissues  are  undifferentiated^  though  potentially  capal)le 
of  differentiation .     In  the  same  way,  the  tissues  of  the  embry- 


FiG.  4. — Cross-section  through  dead  wood-like  cells  from  the  underground  stem  of  a 
fern  (Ptei'is  aquilina) .  The  walls  are  uncommonly  thick  and  the  protoplasm  has 
disappeared.  The  channels  shown  served  in  life  to  keep  the  cells  in  vital  con- 
nection,   (x  450.) 

onic  human  hand  are  imperfectly  differentiated,  and  at  a  very 
early  stage  are  undifferentiated. 

Tissues  composed  of  Cells.  Finally,  microscopical  examina- 
tion shows  every  tissue  to  be  composed  of  minute  parts  known 
as  cells,  which  are  nearly  or  quite  shnilar  to  one  another  through- 
out the  whole  tissue,  and  form  the  ultimate  units  into  which  the 
tissues  and  organs,  and  hence  the  whole  organism,  become  more 
or  less  perfectly  divided,  somewhat  as  a  nation  is  divided  into 
states  and  these  into  counties  and  townships. 


CELLS.  13 

It  will  be  sliown  beyond  that  these  ultimate  units  or  cells 
possess  everywhere  the  same  fundamental  structure;  but  they 
differ  immensely  in  form,  size,  and  mode  of  action,  not  only  in 
different  animals  and  plants,  but  even  in  different  parts  of  the 
same  individual.  As  a  rule,  the  cells  of  any  given  tissue  are 
closely  similar  one  to  another  and  are  devoted  to  the  same  func- 
tion, but  differ  from  those  of  other  tissues  in  form.,  size,  arrange- 
ment, and  especially  in  function.  Indeed,  the  differences  be- 
tween tissues  are  merely  the  outcome  of  the  differences  between 
the  cells  composing  them.  The  skin  of  the  hand  differs  in  ap- 
pearance and  uses  from  the  muscle  which  it  covers,  because  skin- 
cells  differ  from  muscle-cells  in  form,  size,  color,  function,  etc. 
Hence  a  tissue  may  be  defined  as  a  groiij)  of  similar  cells  hav- 
ing a  similar  function.^  As  a  rule,  each  organ  consists  of 
several  such  groups  of  cells  or  tissues,  but,  as  stated  above,  young 
organs  are  nearly  or  quite  homogeneous ;  that  is,  all  of  the  cells 
are  nearly  or  quite  alike.  It  is  only  when  the  organ  grows 
older  that  the  cells  become  different  and  arrange  themselves  in 
different  groups, — a  process  known  as  the  differentiation  of  the 
tissices.  In  the  case  of  some  organs — for  instance  the  leaf  of  a 
moss — the  cells  remain  permanently  nearly  alike,  somewhat  as 
in  the  embryonic  condition,  and  the  whole  organ  consists  of  a 
single  tissue.    . 

What  has  been  said  thus  far  applies  only  to  higher  plants 
and  animals.  But  it  is  an  interesting  and  suggestive  fact  that 
there  are  also  innumerable  isolated  cells,  both  vegetal  and 
animal,  which  are  able  to  carry  on  an  independent  existence  as 
one-celled  plants  or  animals.  Physiologically  these  must  cer- 
tainly be  regarded  as  individuals ;  but  it  is  no  less  certain  that 
they  are  equivalent,  morphologically,  to  the  constituent  cells  of 
ordinary  many-celled  organisms.  It  will  appear  hereafter  thai 
the  study  of  such  unicellular  organisms  forms  the  logical  ground- 
work of  all  biological  science.     (See  p.  157.) 

Since  organisms  may  be  resolved  successively  into  organs, 
tissues  and  cells,  it  is  evident  that  cells  must  contam  living 
matter.  And  a  cell  may  be  defined  as  a  small  mass  of  living 
TTiatter  either  living  apart  or  forming  one  of  the  ultimate  units 

*  Tissues  frequently  contain  matters  deposited  between  cells  ;   but  these 
Lave  usually  been  directly  derived  from  tlie  cells,  and  vary  as  the  cells  vary. 


14 


THE  STRUCTURE  OF  LIVING   THINOS. 


'i-X 


of  an  organism.     The  cell  is  an  "  organic  individual  of  the  first 
order. "^"^     (Lang.) 

Living  and  Lifeless  Matter  in  the  Living  Organism.  Since  our 
own  bodies  and  those  of  lower  animals  and  of  plants  are  com- 
posed of  matter,  it  may  be  supposed,  from  what  has  been  said 
in  the  last  chapter,  that  they  are  composed  of  li\dng 
matter.  This,  however,  is  true  only  in  part.  It  is 
strictly  true  that  every  plant  or  animal  contains  living 
matter,  but  a  little  reflection  will  show  that  it  contains, 
lifeless  matter  also.  In  the  human  body  lifeless  mat- 
ter is  found  in  the  hairs,  the  ends  of  the  nails,  and 
the  outer  layers  of  the  skin, — structures  which  are 
not  simply  devoid  of  feeling,  as  every  one  knows  them 
to  be,  but  are  really  lifeless  in  every  sense,  although 
forming  part  of  a  living  body.  Nor  is  lifeless  mat- 
ter confined  to  the  exterior  of  the  body.  The  mineral 
matter  of  the  bones  is  not  alive;  and  this  is  true, 
though  less  obviously,  of  many  other  parts,  such  as- 
the  liquid  basis  or  plasma  of  the  blood,  the  fat  (which 
is  never  wholly  absent),  and  various  other  forms  of  mat- 
ter occurring  in  many  parts  of  the  body. 

In  lower  animals  examples  of  this  truth  occur  on 
every  hand.     The  calcareous  shells  of  animals  like  the 

snail  and  the  oyster ;  the  skeletons  of 
corals  and  sponges ;  the  hard  outer  crust 
'f^A  of  insects,  lobsters,  and  related  animals ; 
the  scales  of  fish  and  reptiles;  the 
feathers,  claws,  and  beaks  of  birds ;  the 
fur  of  animals — these  are  a  few  of  the 
countless  instances  of  structures  com- 
posed wholly  or  in  part  of  lifeless  mat- 
FiG.5.(AfterRanvier)-Mns-tej.    ^j^j^i^    nevertheless  enter  into  the 

cle-ceUs.  A,  from  the  mtes-         ^  ^  ^ 

tine  of  a  dog,  in  cross-sec-  Composition  of  living  animals. 

tion;  B,  single  isolated  cell,  A,^^^,,^    -.^l^^-t       ^'^         £     j. 

from  the  intestine  of  a  rab-        Among   plants   like    facts  are   even 

bit,  viewed   from   the  side,  more   COllSpicUOUS.         'No   OUC    Cail    doubt 

that  the  outer  bark  of  an  oak  is  devoid 
of  life.  The  heart-wood  of  a  tree  is  entirely  dead,  and  even 
in  the  so-called  live  wood,  through  which  the  sap  flows,  not  only 
is  the  solid  part  of  the  wood  lifeless,  but  also  the  sap  itself. 


LIFELESS  MATTER  BETWEEN  CELLS. 


15 


Fig.  6    (After  Schafer.) — Human  cartilage  (from  head  of  metatarsal  bone),    c,  cells; 

ni,  lifeless  matrix.    (X  600.) 


Fig.  7.  (Modified  from  Ran vier.)— Blood  of  frog,  showing  two  forms  of  eells  (cor- 
puscles), one  flattened  and  oval,  one  branched,  floating  in  the  lifeless  plasma. 
(X650.) 


16 


THE  STRUCTURE  OF  LIVING   THINGS. 


Lifeless  Matter  in  the  Living  Tissues.  In  the  tissues  the  Hv- 
ing  cells  are  seldom  in  contact  one  with  another,  but  are  more  or 
less  completely  separated  by  partitions  of  lifeless  matter.  This 
may  be  seen  in  a  section  through  some  rapidly  growing  organ 
like  a  young  shoot  (Fig.  1).  The  wliole  mass  is  formed  of 
nearly  similar,  closely  crowded  units  or  cells  separated  by  very 
narrow  partitions.  Each  cell  consists  of  a  mass  of  granular, 
viscid,  living  substance  known  2,'^  jprotoijlasm^  and  a  more  solid, 
rounded  body,  the  micleus. 

In  such  a  group  of  cells  no  tissues  can  be  distinguished ;  or, 
rather,  the  whole  mass  consists  of  a  "single  tissue  (meristem), 
wliich  is  almost  entirely  composed  of  living  matter  (protoplasm). 
In  older  tissues  the  partitions  often  increase  in  thickness,  as 
shown  in  Fig.  2.  In  every  case  the  ])artitwns  are  composed  of 
lifeless  matter  which  has  heen  niannfactitred  and  deposited  hy 
the  living  jprotoplasni  constituting  the  hodies  of  the  cells.  In 
still  older  parts  of  the  plant  certain  of  the  lifeless  walls  may 
become  extremely  thick,  the  protoplasm  entirely  disappears,  and 

the  whole  tissue  (wood)  consists  of 
Kf  eless  matter  enclosing  spaces  filled 
with  air  or  water  (Figs.  3  and  4). 

Among  animals  analogous  cases 
are  common.  The  nuiscles  of  the 
small  intestine,  for  instance,  (Fig. 
5,)  consist  of  bundles  of  elongated 
cells  {fihres)  eacli  of  which  is  com- 
posed of  living  matter  surrounded 
by  a  very  tliin  covering  (sheath)  of 
lifeless  matter.  In  cartilao^e  or 
gristle,  which  covers  tlie  ends  of 
many  bones  (Fig.  6),  the  oval  cells 
are  very  widely  separated  by  the 
deposition  between  them  of  large 
quantities  of   solid   lifeless  matter 

Fig.  8.  (Modified  from  Schenk.)— Sec-  /?  •  i     <       •        i  ^^ 

tion  of  bone  from  the  human  femur  lOrmmg    what     IS     kuown     aS     the 

showing  the  living  branching  bone-  matrix.         In     blood     (Fig.      7)     the 
cells  lying   inthe  bony  life  less  ma-  n    ,,  i  .  in/ 

trix.   Diagramatic.  flattened  ,  or    irregular    cells   {cor- 

jpuscles)  are  separated  by  a  lifeless 
fluid  {j>lasind)  in  which  they  float.     In  bone  (Fig.   8)  the  cells 


^J.::;^]:^:-:  '■^t?^l^;^^i^:'^-;/i;^^:[^iyi^ 


LIFELESS  MATTER    WITHIN  CELLS. 


17 


have  a  branching,  irregular  form,  and  are  separated  by  solid 
calcareous  matter  whicli  is  unmistakably  lifeless.  These  ex- 
amples show  that  the  lifeless  matters  of  the  body  often  occur  in 
the  form  of  deposits  between  living  cells  by  which  tliey  have 
been  produced.  In  all  such  cases  the  embryonic  tissue  consists 
at  first  of  living  cells  in  direct  contact,  or  separated  by  only  a 
very  snmll  quantity  of  lifeless  matter.  In  later  stages  the 
cells  may  manufacture  additional  lifeless  substance  which 
appears  in  the  form  of  firm  partition-walls  between  the  cells, 
or  as  a  matrix,  solid  or  liquid,  in  which  the  cells  lie.  AVhen 
solid  walls  are  present  they  are  often  perforated  by  narrow  clian- 
nels  through  which  the  protoplasmic  cell-bodies  remain  in  con- 
nection.     (See  Figs.  4,  8,  and  50.) 

Lifeless  Matter  within  Living  Cells.  Equally  important  with 
the  deposit  of  lifeless  matter  hetioeen  cells  is  the  formation  of  Kfe- 
less  matter  within  cells,  either  {ct)  by  the  deposition  of  various  sub- 
stances in  the  protoplasm,  or  (b)  by  the  direct  transformation  of 
the  whole  mass  of  protoplasm.     Examj)les  of  the  first  kind  are 


Fig.  9.— a  group  of  cells  from  the  stem  of  a  geranium 
(Pelargonium),  showing  lifeless  substances  (starch 
and  crystals)  within  the  protoplasm.  As  in  Fig.  2, 
each  cell  contains  a  large  central  vacuole,  filled 
with  sap  ;  c,  groups  of  crystals  of  calcium  oxalate  ; 
i.e.,  intercellular  space ;  n,  nucleus ;  s,  granules  of 
starch,     (x  300.) 


Fig.  10.  (After  Ranvier.)  — 
Group  of  "adipose  cells" 
from  the  tissue  beneath  the 
skin  ("  subcutaneous  con- 
nective tissue")  of  an  em- 
bryo calf,  showing  drops  of 
fat  in  the  protoplasm.  /,  fat- 
drops  (black)  ;  n,  nuclei 
(X550.) 


mineral  crystals  (Fig.  9),  grains  of  starch  (Fig.  9),  drops  of 
water,  and  many  other  substances  found  witliin  tlie  cells  of 
plants.      Among  animals  drops  of  fat  (Fig.  10)  and  calcareous 


18 


THE  STRUCTURE  OF  LIVING   THINGS. 


or  siliceous  deposits  are  similarly  produced.  Indeed,  there  is 
scarcely  any  limit  to  the  number  of  lifeless  substances  which 
may  thus  appear  within  the  cells  both  of  plants  and  animals. 

The  second  case  is  of  less  importance,  though  of  common 
occurrence.  A  good  examj^le  is  found  in  the  lining  membrane 
of  the  cesopliagus  of  the  dog  (Fig.  11),  which  like  the  human 
skin  is  almost  entirely  made  up  of  closely  crowded  cells.     Those 


P 


Fig.  11.— Section  through  the  inner  coat  of  the  gullet  of  a  dog,  showing :  p,  living 
cells  of  the  deeper  layers ;  s,  lifeless  cells  of  the  superficial  layers;  71,  nucleus. 

in  the  deepest  part  consist  chiefly  of  living  protoplasm  very 
similar  to  that  of  the  young  pine  shoot  (compare  Fig.  1). 
Above  them  the  cells  gradually  become  flattened  until  at  the 
surface  they  have  the  form  of  flat  scales.  As  the  cells  become 
flattened  their  substance  changes.  The  protoplasm  diminishes 
in  quantity  and  dies;  so  that  near  the  surface  the  cells  are 
wholly  dead,  and  finally  fall  oft'.  In  a  similar  manner  are 
formed  the  lifeless  parts  of  nails,  claws,  beaks,  feathers,  and 
many  related  structures.  A  hair  is  composed  of  cells  essentially 
like  those  of  the  skin.  At  the  root  of  the  hair  they  are  alive, 
but  as  they  are  pushed  outwards  by  continued  growth  at  the 
root,  they  are  transformed  bodily  into  a  dead,  horny  substance 
forming  the  free  portion  of  the  hair.  Feathers  are  only  a  com- 
plicated kind  of  hair  and  are  formed  in  the  same  way. 

It  is  a  significant  fact  that  the  quantity  of  lifeless  matter  in 
the  organism  tends  to  increase  with  age.  The  very  young  plant 
or  animal  probably  possesses  a  maximum  proportion  of  proto- 
plasm, and  as  life  progresses  lifeless  matter  gradually  accumulates 
within  or  about  it, — sometimes  for  support,  as  in  tree-trunks  and 


THE  STRUCTURE  OF  LIVING   THINGS.  19 

bony  skeletons ;  sometimes  for  protection  as  in  oyster-  and  snail 
shells ;  sometimes  apparently  from  sheer  inability  on  the  part  of 
the  protoplasm  to  get  rid  of  it.  Thns  we  see  that  youth  is  lit- 
erally the  period  of  life  and  vigor,  and  age  the  period  of  com- 
parative lifelessness. 

Summary.  The  bodies  of  higher  animals  and  plants  are 
subdivided  into  various  parts  {organs)  having  different  structure 
and  functions.  These  mav  be  resolved  into  one  or  more  tissues^ 
each  of  which  consists  of  a  mass  of  similar  cells  (or  their  deriva- 
tives) having  a  similar  function.  The  cells  are  small  masses  of 
living  matter,  or  protoplasm,  which  dej)Osit  more  or  less  lifeless 
matter  either  around  (outside)  them  or  within  their  substance. 
In  the  former  case  the  protoplasm  may  continue  to  live,  or  it 
may  die  and  be  absorbed.  In  the  latter  case  it  may  likewise  live 
on  for  a  time,  or  may  die,  either  disappearing  altogether  or  leav- 
ing behind  a  residue  of  lifeless  matter. 

The  Organism  as  a  Whole.  Up  to  this  point  we  have  con- 
sidered living  organisms  from  an  anatomical  and  analytical  stand- 
point, and  have  observed  their  natural  subdivisions  into  organs, 
tissues,  and  cells.  We  have  now  only  to  remark  that  these  parts 
are  mutually  interdependent,  and  that  the  organism  as  a  whole 
is  greater  than  any  of  its  parts.  Precisely  as  a  chronometer  is 
superior  to  an  aggregate  of  wheels  and  springs,  so  a  living  organ- 
ism is  superior  in  the  solidarity  of  its  parts  to  a  mere  aggregate  of 
organs,  tissues,  and  cells.  We  shall  soon  see  that  in  the  living 
body  these  have  had  a  common  ancestry  and  still  stand  in  the 
closest  relationship  both  in  respect  to  structural  continuity  and 
community  of  interest. 


CHAPTEK  III. 


PROTOPLASM  AND   THE   CELL. 


It  lias  been  sliown  in  the  last  chapter  that  life  is  inherent  in 
a  peculiar  substance,  protoiylasm^  occurring  in  definite  masses  or 
cells.  In  other  words,  protoj)lasm  is  the  physical  basis  of  life, 
and  the  cell  is  the  ultimate  visible  structural  unit.  Protoplasm 
and  the  cell  deserve  therefore  the  most  careful  consideration; 
but  because  of  the  technical  difficulties  involved  in  their  study 
only  such  characteristics  as  are  either  obvious  or  indispensable  to 
the  beginner  will  here  be  dwelt  upon. 

Historical  Sketch.  Organs  and  tissues  are  readily  visible,  but 
in  order  to  resolve  tissues  into  cells  something  more  than  the 
naked  eye  was  necessary.  The  compound  microscope  came  into 
use  about  1650,  and  in  1665  the  English  botanist  Robert  Hooke 
announced  that  a  familiar  vegetal  tissue,  cork,  is  made  up  of 
''^little  hoxes  or  cells  distinct  from  one  another  ^  Many  other 
observers  described  similar  cells  in  sections  of  wood  and  other 
vegetal  tissues,  and  the  word  soon  came  into  general  use.  It 
was  not  until  1838,  however,  and  as  a  consequence  of  a  most 
important  improvement  in  the  compound  microscope,  viz.,  the 
invention  of  the  achromatic  objective,  that  cellular  structure 
came  to  be  recognized  as  an  invariable  and  fundamental  charac- 
teristic of  li\dng  bodies.  At  this  time  the  botanist  Schleiden 
brought  forward  proof  that  the  higher  plants  do  not  simply  con- 
tain cells  but  are  wholly  made  up  of  them  or  their  products ;  and 
about  a  year  later  the  zoologist  Schwann  demonstrated  that  the 
same  is  true  of  animals.  This  great  generalization,  known  as 
the  ' '  cell-theory ' '  of  Schleiden  and  Schwann^  laid  the  basis  for 
all  subsequent  biological  study.  The  cell-theory  was  at  first  de- 
veloped upon  a  purely  morphological  basis.     Its  application  to 

the  phenomena  of  physiological  action  was  for  a  time  retarded 

20 


HISTORY  OF  ''CELL"   AND   ''PROTOPLASM."  21 

hv  the  misleading  character  of  the  term  "cell."  The  word  itself 
shows  that  cells  were  at  first  regarded  as  cavities  (hke  tlie  cells 
of  a  honeycomb  or  of  a  prison)  surrounded  by  solid  walls ;  and 
even  Schleiden  and  Schwann  had  no  accurate  conception  of  their 
true  nature.  Soon  after  the  promulgation  of  the  cell-theorv, 
however,  it  was  shown  that  both  the  walls  and  the  cavity  miirlit 
be  wanting,  and  that  therefore  the  remaining  portion,  namely, 
the  protoplasm  with  its  nucleus,  must  be  the  active  and  essential 
part.  The  cell  was  accordingly  defined  by  Yirchow  and  Max 
Schultze  as  "a  mass  of  protoplasm  surrounding  a  nucleus,"  and 
in  this  sense  the  word  is  used  to-day.  ^^  The  word  cell  became 
thereafter  as  inappropriate  as  it  would  be  if  applied  to  the  honey 
within  the  honeycomb  or  to  the  living  prisoner  in  a  prison -cell, 
Nevertheless,  by  a  curious  conservatism,  the  term  was  and  is  re- 
tained to  designate  these  structures  whether  occurring  in  masses, 
as  segments  of  the  plant  or  animal  body,  or  leading  independent 
lives  as  unicellular  organisms. 

Protoplasm  was  observed  long  before  its  significance  was 
understood.  The  discovery  of  its  essential  identity  in  plants  and 
animals  and,  ultimately,  the  general  recognition  of  the  extreme 
importance  of  the  role  which  it  everywhere  plays,  must  be  reck- 
oned as  one  of  the  greatest  scientific  achievements  of  this  cen- 
tury. It  was  Dujardin  w^ho  in  1835  first  distinctly  called  atten- 
tion to  the  importance  of  the  ''primary  animal  substance"  or 
''sarcode"  which  forms  the  bodies  of  the  simplest  animals. 
Without  clearly  recognizing  this  substance  as  the  seat  of  life,  or 
using  the  word  protoplasm,  he  nevertheless  described  it  as  en- 
dowed with  the  powers  of  spontaneous  movement  and  con- 
tractility. The  word  protoplasm  (rrpcSTOs,  first;  TrXdajua, 
form)  was  apparently  first  used  for  animal  substance  by  Purkinje 
in  1839-40,  and  next  by  H.  von  Mold,  in  18-16,  to  designate 
the  granular  viscid  substance  occurring  in  plant-cells,  although 
both  workers  were  ignorant  of  its  full  significance.  In  1850 
Cohn  definitely  maintained  not  only  that  animal  sarcode  and 
vegetal  protoplasm  were  essentially  of  the  same  nature,  but 
also  that  this  substance  is  the  real  seat  of  vitality  and  hence  to 
be  regarded  as  the  physical  basis  of  life.      To  Max    Schultze 

*  It  is  possible  that  in  some  of  the  lowest  and  simplest  organisms  even  tbe 
nucleus  may  be  wanting  as  a  distinctly  differentiated  body.     See  p.  193. 


22  PROTOPLASM  AND   THE  CELL. 

(1860)  is  generally  assigned  the  credit  of  having  finally  placed 
this  conclusion  upon  a  secure  basis ;  and  by  him  the  meaning  of 
the  word  Protoplasm  was  so  extended  as  to  include  all  living 
matter,  whether  animal  or  vegetal.  In  this  sense  the  word  is 
now  universally  employed. 

Appearance  and  Structure.  Protoplasm  and  cells  differ 
gi'eatly  in  appearance  in  different  plants  and  animals,  as  well  as 
in  different  parts  and  different  stages  of  development  of  the 
same  individual.      The  appearance  of  protoplasm  and  the  consti- 

,„  ,.  tution  of  the  cell  are  as  a  rule 

m^        >!^>r^\v-.r.T^-sJV  most  easily  made  out  in  very 

young  structures,  such  as  the 
eggs  of  some  animals  or  in 
the  cells  of  young  vegetal 
slioots.  The  egg  of  the  star- 
fish, for  example,  (Fig.  12),  is 
I^^      ^<^'l'- '':■  "^';'V'^' ^'^?-''>^     a  sin2:le  isolated  cell  of  nearly 

tyj^ical   form    and   structure. 
It  is  a  minute,  nearly  spheri- 
FiG.  i2.-siightiy  diagrammatic  figure  of  gal  bodv  U^  inch   diameter) 

the  egg  or  ovum  of  a  star-fish,  showing  the  .  ,  .   '        , 

structure  of  a  typical  ceU.    w,  membrane;  m   wllich   three    parts   may  be 

n,  nucleus;  p,  protoplasm  (cytoplasm).         disthlguislied,     viz.  I    (1)      the 

cell-hody^  which  forms  the  bulk  of  the  cell ;  (2)  the  nucleus^  a 
rounded  vesicular  body  suspended  in  the  cell -body ;  (3)  the  Tnein- 
hrane  or  cell-icall^  which  immediately  surrounds  the  cell-body. 
Of  these  three,  the  nucleus  and  cell- body  are  mainly  composed 
of  protoplasm,  while  the  membrane  is  a  lifeless  dej)Osit  upon  the 
exterior.  The  protoplasm  of  the  cell-body  is  generally  called 
cell-plasm,  or  cytoj>lcts7}i^  that  of  the  nucleus  nucleoplasm'j  that 
is,  the  living  matter  of  the  cell  is  differentiated  into  two  different 
but  closely  related  forms  of  protoplasm,  cytoplasm  and  nucleo- 
plasm. 

The  Cytoplasm  appears  as  a  clear  semifluid  or  viscid  sub- 
stance, containing  numerous  minute  granules  and  of  a  watery 
appearance,  though  it  shows  no  tendency  to  mix  with  water. 
Under  very  high  powers  of  the  microscope,  especially  after  treat- 
ment with  suitable  reagents,  the  clear  substance  is  found  to  have 
a  definite  structure,  the  precise  nature  of  which  is  in  dispute. 
By  some  observers  it  is  described  as  a  fibrous  meshwork  or  retic- 


THE  MINUTE  ANATOMY  OF  THE  CELL. 


23 


Ilium,  like  a  sponge;  l)y  others  as  more  neai-ly  like  an  emulsion 
or  foam,  consisting  of  a  more  solid  framework  enclosing  innu- 
merable minute  separate  spherical  cavities  tilled  with  li(piid ;  hy 
others  still  as  comj)osed  of  unbranched  threads  running  in  all 
directions  through  a  more  liquid  basis ;  but  its  real  nature  is  still 
unknown. 

It  is  evident  that  the  visible  structure  of  protoplasm  gives  no 
hint  of  its  marvellous  powers  as  the  seat  of  vital  action,  and  we 
are  therefore  compelled  to  infer  that  it  is  endowed  with  a  chemi- 
cal and  molecular  constitution  extremely  complex,  and  probably 
far  exceeding  in  comj)lexity  that  of  any  lifeless  substance. 

The  Nucleus  is  a  rounded  body  suspended  in  the  cell-sul)- 
stance ;  it  is  distinguishable  from  the  latter  by  its  higher  refrac- 
tive powder,  and  by  the  intense  color  it  assumes  when  treated 
with  staining  fluids.  It  is  surrounded  by  a  very  thin  membrane, 
and  consists  internally  of  a  clear  substance  {acliromatin)^  through 
which  extends  an  irregular  network  of  fibres  (cliromath}).  It 
is   especially  these  fibres  which  are  stained  by  dyes.     In   the 


Fig.  13.  (After  Sachs.)— Young  growing  cells  from  the  extreme  tip  of  a  stonewort 
{Oiara),  m,  membrane;  ?i,  nuclei;  p,  protoplasm;  v,  vacuole  filled  with  sap. 
(X550O 

meshes  of  the  network  is  suspended  in  many  cases  a  second 
rounded  body  known  as  the  nucleolus,  which  stains  even  more 
deeply  than  the  network  itself. 

The  Membrane  or  Wall  of  the  cell  forms  a  rather  thick  sac, 


24  PROTOPLASM  AND   THE  CELL. 

composed  of  a  soft,  lifeless  material  closely  surrounding  the  cell 
substance."^ 

As  a  second  example  we  clioose  tlie  growing  jDoint  of  a  com- 
mon water-plant  (Cliarci)^  Fig.  13.  This  structure  is  composed 
of  cells  which  are  more  or  less  angular  in  outline  as  a  result  of 
mutual  pressure,  but  show  otherwise  an  unmistakable  similarity 
to  the  egg-cell  just  described.  They  differ  mainly  in  the  fact 
that  the  protoplasm  of  the  larger  cells  contains  rounded  cavities, 
known  as  vacuoles^  filled  with  sap  ((v) ;  also  in  the  chemical  com- 
position cf  the  cell-walls  (here  consisting  of  "cellulose,"  a  sub- 
stance of  rare  occurrence  among  animals). 

Origin  of  Cells  and  Genesis  of  the  Body.  The  body  of  every 
higher  plant  or  animal  arises  from  a  single  germ -cell  ("  ^ggt^"^ 
"  spore,"  etc.)  more  or  less  nearly  similar  to  that  of  the  star- 
fish, described  above,  and  originally  forming  a  part  of  tlie  parent 
body.  The  germ-cell,  therefore,  in  spite  of  endless  variations  in 
detail,  shows  us  the  model  after  which  all  others  are  built ;  for 
it  gives  rise  to  all  the  cells  of  the  body  by  a  continued  j^rocess 
of  segmentation  as  follows  : 

The  first  step  (Fig.  14)  consists  in  the  division  of  the  Qg^g 
into  two  similar  halves,  which  differ  from  the  original  cell  only 
in  lacking  membranes,  both  being  surrounded  by  the  membrane 
of  the  original  cell.  Each  of  the  halves  divides  into  two,  mak- 
ing four  in  all ;  these  again  into  two,  making  eight,  and  so  on 
throughout  the  earlier  part  of  the  development.  By  this  jDrocess 
(known  as  the  cleavage  or  segtnentation  of  the  ^g^  the  germ- 
cell  gives  rise  successively  to  2,  4,  8,  16,  32,  64,  etc.,  de- 
scendants, forming  a  primitive  body  composed  of  a  mass  of 
nearly  similar  cells,  out  of  which,  by  still  further  division  and 
growth,  the  fully-formed  body  of  the  future  animal  is  to  be 
built  up.  These  cells  are  only  slightly  modified,  but  differ  in 
most  animals  from  the  typical  germ-cell  in  having  at  first  no  sur- 
rounding membranes.  The  membrane  of  the  original  germ- 
cell  meanwhile  disappears. 

^  The  word  cell  lias  been  used  in  Chap.  I  and  elsewhere  to  denote  the 
living  matter  within  the  membrane,  the  latter  being  considered  a  product  of 
the  cell  rather  than  an  integral  part  of  it.  It  is  more  usual  to  include  the 
membrane  in  a  definition  of  the  cell,  and  as  a  matter  of  convenience  it  is  so 
included  here. 

pupLRii  imtARr 


DEVELOPMENT  AND  DIFFERENTIATION  OF  CELLS.       25 

The  enibryoiiic  body  or  emhryo  of  every  liiglier  })lant  and  ani- 
mal is  derived  from  the  germ-cell  by  a  process  essentially  like  that 
just  described,  though  both  the  form  of  the  cells  and  the  order  of 
division  are  usually  more  or  less  irregular.      In  animals  the  cells 


Fig.  14.— Cleavage  or  segmentation  of  an  ovum,  showing  successive  division  of  the 
germ-cell  (a)  into  two  (b),  four  (c),  and  eight  (r/).  Later  stages  are  shown  ate 
and  /.  The  first  four  figures  are  diagrammatic ;  e  and  /  are  after  Hatschek's  fig- 
ures of  the  development  of  a  very  simple  vertehrate  {AmpMoxxui). 

thus  formed  are  usually  naked  at  first,  though  they  often  ac- 
quire a  membrane  in  later  stages.  Among  plants,  on  the  con- 
trary, the  cells  usually  possess  membranes  from  the  first,  prob- 
ably because  their  need  for  a  firm  outer  support  is  greater  than 
the  need  for  free  movement  demanded  by  animals. - 

Modification  of  the  Embryonic  Cells.  Differentiation.  The 
close  similarity  of  the  embryonic  cells  does  not  long  persist.  As 
development  proceeds,  the  cells  continually  increasing  in  munber 
by  division  become  modified  in  difl:'erent  ways,  or  fliffereiifiatefl^ 
to  fit  them  for  the  many  different  kinds  of  work  which  they  have 
to  do.  Those  which  are  to  become  muscle-cells  gradually  assume 
an  entirely  different  form  and  structure  from  those  which  are  to 
become  skin-cells;  and  the  future  nerve-  or  gland-cells  take 
on  still  other  forms  and  structures.  The  embrvonic  cells  are 
gradually  converted  into  the  elements  of  the  difi'ereiit  tissues — 
this  process  being  the  differentiation  of  the  tissues  which  has 


*  For  a  more  precise  account  of  cell-division  see  p.  83. 


26  PROTOPLASM  AND   THE  CELL. 

already  been  mentioned  on  p.  11 — and  are  in  tins  way  enabled 
to  effect  a  physiological  division  of  labor. 

Tlie  variations  in  form  and  structure  which  thus  appear  are 
endlessly  diversified.  Cells  may  assume  almost  any  conceivable 
form,  and  there  are  even  cells  (e.g.,  Amoebce^  or  the  colorless 
corpuscles  of  the  blood)  which  continually  change  their  form 
from  moment  to  moment.  The  variations  in  structure  may  in- 
volve any  or  all  of  the  three  characteristic  j^arts  of  the  typical 
cell,  being  at  the  same  time  accompanied  by  variations  of  form. 
It  is  easy  to  understand,  therefore,  how  cells  may  vary  endlessly 
in  appearance,  wliile  conforming  more  or  less  closely  to  the  same 
general  type. 

Meanwhile  the  protoplasm  itself  undergoes  extensive  altera- 
tion.    Even  in  young  cells,  or  in  the  germ-cell  itself,  it  may 
=ssF7;;^Sn:i?^fSmn^iv-^  contain  an  admixture  of  other  substances, 

lS^li;,;;;;;;;iuu,',i^;;;;;;:;;iii;;  and   these    may    entirely   change    their 

ii^/ji^a,  iiiHWjiiifiiv^i  :  ,,,,,1^  character  or  (as  is  especially  common  in 

'IttfSiitifHS^^  plant-cells)    may    become    more    abun- 

dant as  the  cell  grows  older,  taking  tlie 
^?^'SI;if;;"  shape  of  fluid,  solid,  or  even  gaseous  de- 


;f'-vi)|]i|ll3'i|-.irV.:,:,-.j,,j:,,_,yf, 


'Prnvtsmm-rrt, ..." 

iivinnwr.) 


;ss!''"^/'"i«u, 


Jf;i.;,-';:,„r,.,rt»:!'PaTfyo,;  'y,!  W'iMiMifflii 


,.„       ,  posits.     Common  examples  of  such  de- 

^^^^^^^maftmn^m         posits  are  drops  of  water,  oil,  and  resin, 
^mmmm0mmmmmm':         granules  oi  piscment,    starcli,   and  solid 


mf^mmmm^mkr  proteid  matters,  and  crystals  of  mineral 

Fig.  15.     (After  Ranvier.)—    ^  im  i    • 

Part  of  a  single  fibre  of  vol-  substaiiccs    like   calciuiii   Oxalate,    phos- 
nntary  muscle  from  the  leg     .^^^  ^^^^  carbonate,   and  siKca.     Bub- 

of  a  rabbit,    p,  protopiasin ;    i:  ' 

71,  nucleus.   (x700.)  blcs  of  gas  somctimcs  appear  in  the  pro- 

toplasm, but  this  is  exceptional.  The  living  substance  itself 
often  changes  in  appearance  as  the  cells  become  differentiated. 
The  protoplasm  of  voluntary  muscles  (Fig.  15)  is  firm,  clear, 
non-granular,  highly  refractive,  and  arranged  in  alternating 
bands  or  stripes  of  darker  and  lighter  substance.  In  some  cases 
(e.g.,  the  outer  portions  of  the  skin,  or  of  a  hair,  as  explained 
in  Chap.  II)  the  modifications  of  tlie  cell-substance  becomes  so 
great  that  both  its  physical  and  chemical  constitution  are  entirely 
altered,  and  it  is  no  longer  protoplasm,  but  some  form  of  lifeless 
matter. 

Protoplasm  in  Action.     "We  may  now  briefly  consider  proto- 
plasm from  the  dynamical  or  physiological  point  of  view.     We 


PROTOPLASMIC  MOVEMENTS. 


27 


know  that  living  tilings  are  the  seat  of  active  changes,  wliicli 
taken  together  constitute  tlieir  life.  In  tlie  last  analysis  tliese 
changes  are  undoubtedly  chemical  actions  taking  ])lace  in  the 
protoplasm,  which  may  or  may  not  produce  visible  i-esults. 
There  is  no  doubt  that  extensive  and  probably  very  complex 
molecular  actions  go  on  in  the  protoplasm  of  young  growing 
cells,  tliough  it  may  appear  absolutely  quiescent  to  the  eye,  even 
under  a  powerful  microscope.  In  other  cases,  the  chemical 
action  produces  perceptible  clianges  in  the  protoplasm, — for  in 
stance,  some  form  of  motion, — just  as  the  invisible  chemical 
action  in  an  electrical  battery  may  be  made  to  produce  visible 
effects  (light,  locomotion,  etc.)  through  the  agency  of  an  electrical 
machine. 

A  familiar  instance  of  protoplasmic  movement  is  the  contrac- 
tion of  a  muscle.  This  process  is  most  likely  a  change  of  molec- 
ular arrangement,  causing  the  nmscle,  while  keeping  its  exact 
bulk,   to  change  its  form,   the  two  ends  being  brought    nearer 

together  (Fig.  16).  The  visible  change 
of  form  is  here  supposed  to  be  due  to  an 
invisible  change  of  molecular  arrange- 
ment, and  this  in  turn  to  be  coincident 
with  chemical  action  taking  place  in  the 
living  substance. 

A  striking  and  beautiful  example 
of  movement  in  protoplasm  occurs  in 
B  the  simple  organism  known  as  Amceha 
(Fig.  S-I,  p.  159).  The  entire  body  of 
this  animal  consists  of  a  mass  of  naked 
protoplasm  enclosing  a  nucleus,  or 
sometimes  two ;   in  other  words,  it  is  a 

FiG.76.-change  of  form  in  a  single  naked  Cell.      The  protoplasm  of 
contracting  muscle.  A,  mus-  qx\  active  Amocha  is  iu  a  statc  of  cease- 

cle  in  the  ordinary  or  extend-  .  ,. 

ed  state;  B,  the  same  muscle  less  movement,  contractmg,  expanding, 
when  contracted.  (Diagram.)  fl^^^jj^g^  ^nd  changing  the  fomi  of  the 

animal  to  such  an  extent  that  it  is  known  as  the  '' Proteus'' 
animalcule.  The  whole  movement  is  a  kind  of  Hux.  A  })ortion 
of  the  protoplasm  flows  out  from  the  mass,  making  one  or  more 
prolongations  {pseudopods)  into  which  the  remainder  of  the 
protoplasm  finally  passes,  so  that  the  whole  body  advances  in  the 


A 


28 


PROTOPLASM  AND   THE  CELL. 


direction  of  the  flow.     If  particles  of  food  be  met  with,  the 

protoplasm  flows  around  them,  and  when  thej  have  been  digested 

within  tlie  body,  the  protoplasm  flows  onward,  leaving  the  refuse 

behind.     Hour  after  hour  and  day  after  day  this  flowing  may 

go    on,   and    there   is   perhaps   no 

more    fascinating    and    suggestive 

spectacle  known   to  the  biologist. 

A  similar  change  of  form  is  ex- 

liibited  by  the  colorless  corjDuscles 

of  amphibian  and  other  blood,  in 

which  it  may  be  observed,  though 

far  less   satisfactorily,    if   Afnoehce 

cannot  be  obtained.    Among  plants, 

protoplasmic  movements  of  perhaps 

equal    beauty   may   be    observed. 

One  of  the  simplest  is  known  as  the 

rotation  of  protoplasm,  wliich  may 


n- 


'?,»•■*■  •••.,'ov  .•.-.'<:"-  <J  ■■:■■*.  0- 


Fig.  17.— a  cell  of  a  stonewort  (NiteU 
la)  showing  the  rotation  of  proto- 
plasm; the  arrows  show  the  direc- 
tion of  the  flow,  m,  membrane  of 
the  cell;  n,  nucleus,  opposite  to 
which  is  a  second  ;  p,  protoplasm ;  v, 
large  central  vacuole  filled  with  sap. 


i 


5>~        .:-;.■  .-V^^i.^:  •,'^:f> 


r 


Fig.  17a.— Two  cells  and  a  part  of  a 
third  from  the  tip  of  a  "leaf"  of  a 
stonewort,  showing  rotation  of  the 
protoplasm  in  the  direction  of  the 
arrows. 


be  studied  to  advantage  in  rather  young  cells  of  stoneworts  {Cliara 
or  Nitella).  These  cells  have  the  form  of  short  or  elongated 
cylinders  which  are  often  pointed  at  one  end  (Fig.  17).      The 


PROTOPLASMIC  MO  VEMENTS. 


29 


protoplasm  is  surrounded  by  a  delicate  membrane  which  thus 
forms  a  sac  enclosing  the  protoplasm.  In  very  young  cells  the 
protoplasm  entirely  tills  the  sac ;  but  as  the  cell  grows  older  a 
drop  of  liquid  appears  near  the  centre  of  the  mass  and  increases 
in  size  until  the  protoplasm  is  reduced  to  a  thin  layer  {jyinmor- 
dial  utricle)^  lining  the  inner  surface  of  the  memljrane  (compare 
Fig.  2).  In  favorable  cases  the  entire  mass  of  protoplasm  is 
seen  to  be  flowing  steadily  around  the  inside  of  the  sac,  as  in- 
dicated by  the  arrows  in  Fig.  IT.  It  moves  upwards  on  one 
side,  downwards  on  the  opposite  side,  and  in  02)posite  directions 
across  the  ends,  forming,  an  unbroken  circuit.  The  flow  is  ren- 
dered more  conspicuous  by  various  granules  and  other  lifeless 
masses  floating  in  the  protoplasm  and  by  the  large  oval  nucleus 
or  nuclei,  all  of  which  are  swept  onward  by  the  current  in  its 
ceaseless  round.  A  similar  rotation  of  protoplasm  occurs  in  many 
other  vegetal  cells,  one  of  the  best  examples  being  the  leaf-cells 
of  Anacharis. 

A  second  and  somewhat  more  intricate  kind  of  movement  in 
vegetal  protoplasm  is  known  as  circulation.  This  differs  from 
rotation  chiefly  in  the  fact  that  the  protoplasm  travels  not  only  in 
a  peripheral  stream  but  also  in  strands  which  run  across  through 
the  central  space  (vacuole)  and  thus  form  a  loose  network.     Cir- 


Fio.  18.— Flower-cluster  {a)  and  single  stamen  (h)  of  a  cultivated  spidervrort  {Trades' 
cantia).    7i,  hairs  upon  the  stamen,    a,  slightly  reduced ;  b,  slightly  enlarged 

dilation  is  well  seen  in  cells  composing  the  hairs  of  various  ])lants, 
such  as  the  common  nettle  (Urtica),  the  spiderwort  (Trades- 


30 


PROTOPLASM  AND  THE  CELL, 


cantia)^  tlie  liolljliock  {Altlima)^  and  certain  sj^ecies  of  gourds 
{Cucxii'ljitci).  It  may  be  conveniently  studied  in  the  liairs  upon 
the  stamens  of  the  cultivated  spiderwort  {Tradescantia).  The 
fiower  of  this  plant  is  shown  in  Fig.  18,  «,  and  one  of  the 
Btamens  with  its  hairs  at  h.     Each  hair  consists  of  a  single  row 

A 


1^. 


;.\^ 


c '  •  i;  ■;'':■*!  •■-  ••■•  .••"-••"<■  •  •  *1 

fl-lI;f;|■:;■!•^?■•^ 


iifl^^vK^^ 


'.^^<t^?=«,,o^ 


^ 


^> '^  ■•:■■■-:*/•-■■  ^  •  ••>  ;->'6-.  6»>i-r 
■  »  ".  *i   \  .V     ;''.,;  ^  ^  ■  /  *.  N  ft:  '*■  **>  ■  /  /T 

.^ . «  -  w  i*  '•••.&."&  -■■',/  '.  ■•  •*,  *  P  ^  '      » 


v^  v-:-;  >->-,;^\',i:^,..^? 


Fig.  19.— Enlarged  cells  of  the  hairs  from  the  stamens  of  the  spiderwort.  A,  five 
cells,  somewhat  enlarged,  protoplasm  not  shown ;  B  and  G,  cells  much  more  en- 
larged, showing  the  circulation  of  protoplasm  as  indicated  by  the  arrows ;  ?i, 
nucleus. 

of  elongated  cells  covered  by  delicate  membranes  and  connected 
by  their  ends.  As  in  Nitella^  the  protoplasm  does  not  fill  the 
cavity  of  the  sac,  but  forms  a  thin  lining  {jprimordial  utricle) 


CILIARY  ACTION. 


81 


on  its  inner  face  (Fig.  19).  From  this  laj'er  delicate  tlirea(l<  of 
protoplasm  reach  into  and  pass  tlirough  tlie  central  cavity,  where 
they  often  branch  and  are  connected  together  so  as  to  tonii  a 
very  loose  network.  The  nuclens  {n)  is  embedded  eitln'i-  in  the 
peripheral  layer  or  at  some  point  in  the  network,  and  the  threads 
of  the  latter  always  converge  more  or  less  regularly  to  it.  In 
active  cells  currents  continually  flow  to  and  fro  throughcjut  tiie 
whole  mass  of  protoplasm.  In  the  threads  of  the  netwoi-k  gran- 
ules are  borne  rapidly  along,  gliding  now  in  one  direction,  now 
in  another;  and  although  the  flow^  is  usually  in  one  direction  in 
any  particular  thread,  no  system  can  be  discovered  in  the  com- 
plicated movements  of  the  whole.  In  the  larger  threads  the 
curious  spectacle  often  appears  of  two  rapid  currents  flowing  m 
opposite  directions  on  opposite  sides  of  the  same  thread.  The 
currents  in  the  thread  may  be  seen  to  join  currents  of  the  pe- 
ripheral layer  which  flow  here  and  there,  but  without  sthe  regu- 
larity observed  in  the  protoplasm  of  Nitella.  The  protoplasmic 
network  also,  as  a  wdiole,  undergoes  a  slow  but  steady  change  of 
form,  its  delicate  strands  slowly  ,^^i^m  ^  gifc 
swaying  hither  and  thither,  while 
the  nucleus  travels  slowly  from 
point  to  point. 

Finally,  we  may  consider  an 
example  of  a  form  of  protoplas- 
mic movement  known  as  ciliary 
action,  which  plays  an  important 
role  in  our  own  lives  and  those 
of  low^er  animals  and  of  some 
plants.  The  interior  of  the  tra- 
chea, or  windpipe,  is  lined  by 
cells  having  the  form  shown  in 
Fiff.  20.  At  the  free  surface  of 
the  cell  (turned  towards  the  cavi-   ^^^  ^  ^^^^J^  Kiein.)-Three  isolated 

ty  of  the  trachea)  the  l)rotoplasm       ciliated  cells  from  the  interior  <.f  the 
/  -  1     •  1    T      X  -1  windpipe  of  the  cat.   r,  the  cilia  at  the 

is  produced    into    delicate   Vlbra-        free  end;  »,  the  nucleus:  />,  the  proto- 

tory   filaments  having  a  sickle-      pi^^m.   (Hitrhiy  magnified.) 
shape  when  bent;   these  are  known  as  cilia  {cilium,  an  eyelash). 
They  are  so  small  and  lash  so  vigorously  as  to  be  nearly, or  quite 
invisible  until  the  movements  are  in  some  way  made  sluggish. 


32  PROTOPLASM  AND   THE  CELL. 

The  movement  is  then  seen  to  be  more  rapid  and  vigorous  in  one 
■direction  than  in  the  other,  all  the  cilia  working  together  like 
the  oars  of  a  row-boat  acting  in  concerted  motion.  By  this 
action  a  definite  current  is  produced  in  the  surrounding  medium 
(in  this  case  the  mucus  of  the  trachea)  flowing  in  the  direction 
of  the  more  vigorous  movement.  In  the  trachea  this  movement 
is  upwards  towards  the  mouth,  and  mucus,  dust,  etc. ,  are  thus 
removed  from  the  lungs  and  windpipe.  In  many  lower  animals 
and  plants,  especially  in  the  embryonic  state,  cilia  are  used  as 
organs  of  locomotion,  serving  as  oars  to  drive  the  organism 
through  the  water.  The  male  reproductive  germs  of  plants  and 
animals  are  also  propelled  in  a  similar  fashion. 

In  all  these  forms  of  vital  action  the  protoplasm  is  visibly  at 
work.  In  most  cases,  however,  no  movements  of  tlie  protoplasm 
in  cells  can  be  detected.  But  it  is  certain  from  indirect  evidence 
that  protoplasm  is  no  less  active  in  those  modes  of  physiological 
action  that  give  no  visible  outward  sign,  as  for  example  in  an 
active  nerve-cell  or  a  secreting  cell.  This  activity  being  molec- 
ular and  chemical  is  beyond  the  reach  of  the  microscope,  but  it 
is  none  the  less  real ;  and  the  play  of  these  invisible  molecular 
actions  is  doubtless  far  more  tumultuous  and  complicated  tlian  the 
visible  movements  of  the  jDrotoplasmic  mass  displayed  in  Nitella 
or  in  a  nettle-hair.  It  is  of  the  utmost  importance  that  the  stu- 
dent should  attain  to  a  full  and  vivid  sense  of  the  reality  and 
energy  of  this  invisible  activity  even  in  protoplasm  which  (as  is 
ordinarily  the  case)  under  the  closest  scrutiny  appears  to  be  abso- 
lutely quiescent. 

The  Sources  of  Protoplasmic  Energy.  Whence  comes  the 
power  required  for  protoplasmic  action,  and  how  is  it  expended? 
The  answer  to  this  question  can  be  given  at  this  point  only  in 
very  general  terms.  It  is  certain  that  protoplasm  works  by 
means  of  chemical  actions  taking  place  in  its  own  substance; 
and  it  is  further  certain  that  these  actions  are,  broadly  speaking, 
processes  of  oxidation  or  combustion;  for  in  the  long  run  all 
forms  of  protoplasmic  action  involve  the  taking  up  of  oxygen 
and  the  liberation  of  carbon  dioxide.  Energy  is  therefore  set 
iree  in  living,  active  protoplasm  somewhat  as  it  is  in  the  com- 
bustion of  fuel  under  the  boiler  of  a  steam-engine,  and  in  this 
process  the  protoplasm,  like  the  coal,  is  gradually  used  up,  disin- 


CHEMICAL  RELATIONS  OF  PROTOPLASM.  83 

tegrates,  and  wastes  away,  giving  oft'  as  waste  matter  tlie  various 
clieniical  products  of  the  combustion,  and  lil)eratin<r  eneri^v  as 
lieat  and  meclianical  work.  Tlie  loss  of  substance  is,  liowever 
continually  made  good  (much  as  the  coal  is  replenished)  1)V  the 
absorption  of  new  substance  in  the  form  of  food,  wliifli  mav 
consist  of  actual  protoplasm,  derived  from  other  living  bein<'-s, 
or  of  substances  convertible  into  it.  These  substances  are  in 
some  unexplained  way  converted  into  protu])lasni  and  tlius 
built  into  the  living  fabric. 

To  this  dual  process  of  waste  (" /I'rt?^*^^^*??/.")  and  repair 
{'''•  anahoUsm^^)  is  applied  the  term  metaholism^  which  must  be 
considered  as  the  most  characteristic  and  fundamental  propertv 
of  living  matter.  It  is  evident  from  the  foregoing  that  meta- 
bolism involves  on  the  one  hand  a  destructive  action  {katahol- 
isiix)  through  whicli  protoplasm  disintegrates  and  energy  is  set 
free,  and  on  the  other  hand  a  constructive  action  (anaholinn) 
whereby  new  proto23lasm  is  built  up  from  the  income  of  food  and 
fresh  energy  is  stored.  It  is  a  most  remarkable  fact  that  as  far 
as  known  the  constructive  action  resulting  in  the  formation  of 
new  protoplasm  never  takes  place  except  through  the  inmiediate 
agency  of  protoplasm  already  existing.  In  otlier  words,  there  is 
no  evidence  that  "spontaneous  generation"  or  the  production 
of  living  from  lifeless  matter  without  the  intluence  of  antecedent 
life  ever  takes  place.  Nor  is  there  any  evidence  that  any  energy 
can  be  "generated,"  but  rather  that  the  vital  energy  of  living 
things  is  only  the  transformed  energy  of  their  food,  and  that 
"vital  force"  having  an  origin  elsewhere  than  in  such  energy 
does  not  exist. 

Chemical  Relations.  We  know  nothing  of  the  precise  chemi- 
cal composition  of  living  protoplasm,  because,  as  has  been  said 
(p.  2),  living  protoplasm  cannot  be  subjected  to  chemical  analy- 
sis without  destroying  its  life.  But  the  results  of  chemical  ex- 
aminations leave  no  doubt  that  the  molecules  of  protoplasm  are 
highly  complex  and  are  probably  separated  from  one  anotlier  by 
layers  of  water. 

A.  Proteids.  It  has  already  been  stated  (}>.  3)  that  tlie 
characteristic  products  of  the  analysis  of  protoplasm  are  the 
group  of  closely -related  substances  known  sls  proteids.  But  pre^- 
teids  form  only  a  small  part  of  the  total  weight  of  any  plant  or 


34 


PROTOPLASM  AND   TUE  CELL. 


animal,  being  always  associated  with  quantities  of  other  sub- 
stances. Even  the  white  of  an  Qgg^  which  is  usually  taken  for 
a  typical  proteid,  contains  only  twelve  per  cent  of  actual  proteid 
matter,  the  remainder  consisting  cliielly  of  water.  The  follow- 
ing table  shows  the  j^ercentage  of  proteids  and  other  matters  in 
a  few  familiar  organisms  and  their  products : 


PROXIMATE    PERCENTAGE    COMPOSITION  OF  SOME  COMMON 

SUBSTANCES.* 

Arranged  according  to  richness  in  Proteids. 


I 


1 
2 

3 
4 
5 
6 
7 
8 
9 

10 
11 
12 
13 
14 
15 
16 
17 
18 
19 
20 
21 
22 
23 
24 
25 
26 
27 


Apples.. 

Indian  corn,  aerial  portion  fresh 

Oysters,  shells  included 

Turnips 

Melons.  ....         

Sweet  potatoes 

Crayfish,  whole 

Irish  potatoes 

Clams,  round,  shells  included... 

Oats,  aerial  part  fresh  

Grass,     "         "  "     

Peas,       "         "         "     

Cow's  milk.  

Flounder,  whole. 

Lobster,  "■ . 

Poplar  and  elm  leaves,  fresh. . . . 

Crab,  whole 

Brook  trout,  whole 

Hen's  eggs,  shells  included 

Mutton  "  chops  "   

Chicken,  whole 

Beef,  heart 

Beef,  liver.. 

Beefsteak,  round,  lean . 

Beans     

Cheese.. 

Cheese  from  skimmed  milk 


Pro- 

Carbo- 

Other 

Water, 

teids. 

hy- 

Fats. 

Sub- 

drates. 

stances. 

84.8 

0.4 

14.3 

0.0 

0.5 

843 

0.9 

13.7 

0.5 

0.6 

15.4 

1.0 

0.6 

0.2 

82  8 

91.2 

1.0 

69 

0.2 

0.7 

95.2 

1.1 

2  5 

0.6 

0.6 

75.8 

1.5 

21.1 

0.4 

1.2 

10.0 

1.9 

0  1 

0.1 

87.9 

75.5 

2.0 

21.3 

0.2 

1.0 

27.3 

2.1 

1.3 

0.1 

69.2 

81.0 

2.3 

15.3 

0.5 

0.9 

75.0 

3.0 

19.9 

0.8 

0.3 

81.5 

3.2 

13.8 

0.6 

0.8 

87.4 

3.4 

4  8 

3.7 

0.7 

27.2 

5.2 

0.0 

0.3 

67.3 

33.0 

5.4 

0.2 

0.5 

60  9 

70.0 

6.0 

22.0 

1.5 

0.5 

34.1 

7.3 

0.5 

0.9 

56.2 

40.3 

9.9 

1.1 

48.7 

65.6 

11.1 

0.5 

10.8 

12.0 

41.3 

12.5 

•  •  •  • 

29.3 

16.9 

42.2 

14.3 

11 

42.4 

53.4 

14.9 

•  •  •  - 

^.8 

6.9 

69  5 

20.1 

3.5 

5.4 

15 

60.0 

20.7 

•  •  ■  • 

8.1 

11.2 

13.7 

23.2 

57.4 

2.1 

3.6 

31.2 

27.1 

2  4 

35.4 

3.9 

41.3 

38.3 

9.0 

6.8 

4.6 

All  proteids  have  nearly  the  same  chemical  composition  and 
similar  physical  properties,  however  different  may  be  the  forms 
of  protoplasm  in  which  they  occur.  The  analysis  of  protoplasm, 
or  ratlier  of  the  proteids  which  are  its  basis,  teaches  us  really 
nothing  of  its  vital  properties,  but  serves  only  to  sliow  the 
chemical  composition  of  the  material  basis  by  which  these  are 
manifested. 

Proteids  are  so  called  from  tlieir  resemblance  to  protein 
(Trpcyrob,  first)^    a   hypothetical    substance    first  described   and 


*  Compiled  chiefly  from  tables  of  food-composition  prepared  by  W.  O.  Atwater 
for  the  Smithsonian  Institution,  though  a  few  examples  have  been  added— viz.- 
numbers  2, 10, 11, 12, 16— from  Johnson's  How  Crops  &row,  N.  Y.,  1883. 


PROTEIDS. 


35 


named  by  ^rulJer.      According  to  Iloppe-Seyler  they  liave  ap- 
proximately  the  following  percentage  composition  : 


From. 
To.... 


C. 


51.5 
54.5 


H. 


6.9 
7.3 


N. 


15.2 
17.0 


O. 


20  9 
Zi  5 


S. 


03 
2.0 


A  small  quantity  of  phosphorus  is  also  very  frerpiently  present. 
Associated  with  these  elements  are  always  small  quantities  of 
various  mineral  substances  wdiicli  remain  as  the  ash  when  proto- 
plasm is  burned ;  but  the  nature  of  their  relations  to  the  other 
elements  is  uncertain.  The  ash  varies  both  in  quantity  and 
chemical  composition  in  different  animals  and  plants.  In  tho 
white-of-egg  the  chief  constituents  of  the  ash  are  potassium  chlo- 
ride (KCl)  and  sodium  chloride  (XaCl),  the  former  being  much 
in  excess.  The  remainder  consists  of  phosphates,  sulphates,  and 
carbonates  of  sodium  and  potassium,  \\i\\\  minute  quantities  of 
calcium,  magnesium,  and  iron,  and  a  trace  of  silicon.  Many 
other  mineral  substances  occur  in  association  witli  other- kinds  of 
proteids,  but  always  in  very  small  proportion.  These  salts  are  in 
some  way  essential  to  the  activity  of  protoplasm,  as  we  know  by 
familiar  experience.  Man,  like  other  animals  and  the  plants, 
requires  certain  mineral  substances  (e.g.  common  salt),  but  we 
have  no  knowledge  of  the  part  these  play  in  protoplasm. 

It  is  important  to  note  the  close  chemical  similarity  of  animal 
and  vegetal  proteids,  because  this  is  one  reason  for  regarding 
vegetal  and  animal  protoplasm  as  essentially  similar  in  other  re- 
spects. The  following  table,  from  Johnson  after  Gorup-Besanez 
and  Ritthausen,  shows  the  percentage  composition  of  various  pro- 
teids, and  proves  that  the  difference  between  vegetal  and  animal 
proteids  is  chemically  no  greater  than  that  between  different 
kinds  of  vegetal  or  different  kinds  of  animal  proteids : 


PERCENTAGE  COMPOSITION 

OF  PROTEIDS. 

C. 

H. 

N.                 O. 

S. 

Animal  albumen 

53.5 
53.4 
5:16 
50  5 
.>t.l 
,>4.3 
52.6 

7.0 
7.1 
7.1 
6.H 
7.3 
7.2 
7.0 

15.5 
15.6 
15.7 
18.0 
16.0 
16. '1 
17.4 

22.4 

22.6 
24.2 
21.5 
20.6 
21.8 

16 

0  0 
1.0 
0.5 
1.1 
1.0 

... 

Vegetal       "         

Animal  casein 

Vejjetal       "     

Animal  (flesh)  fibrin 

Vegetal  (wheat)    "    

Animal  (blood)      "      

36  PROTOPLASM  AND   THE  CELL. 

There  is  a  corresponding  likeness  in  the  general  properties  and  reactions 
of  proteids.  They  are  colloidal  or  non-diffusible,  i.e.,  they  will  not  pass 
through  the  membrane  of  a  dialyser,  or  only  with  great  difficulty  ;  they 
are  rarely  crystalline  ;  they  rotate  the  plane  of  polarized  light  to  the  left. 
Though  not  all  soluble  in  water,  they  may  be  dissolved  by  the  aid  of  heat 
in  strong  acetic  acid  and  in  caustic  alkalies,  but  are  insoluble  in  cold  ab- 
solute alcohol  and  in  ether.  They  may  be  precipitated  from  solution  by 
strong  mineral  acids,  etc.  Many  proteids  are  precipitated  by  heat  (a  pro- 
cess which  is  called  coagulation)  ;  and  it  is  worthy  of  note  that  tempera- 
tures which  produce  coagulation  of  proteids  (40°  — 75°  C.)  produce  also  the 
death  of  most  organisms.  "Amongst  the  organic  proximate  principles 
which  enter  into  the  composition  of  the  tissues  and  organs  of  living  beings, 
those  belonging  to  the  class  of  proteid  or  albuminous  bodies  occupy  quite 
a  peculiar  place  and  require  an  exceptional  treatment,  for  they  alone  are 
never  absent  from  the  active  living  cells  which  we  recognize  as  the  pri- 
mordial structures  of  animal  and  vegetable  organisms.  In  the  plant,  whilst 
we  recognize  the  wide  distribution  of  such  constituents  as  cellulose  and 
chlorophyl,  and  acknowledge  their  remarkable  physiological  importance, 
we  at  the  same  time  are  forced  to  admit  that  they  occupy  altogether  a 
different  position  from  that  of  the  proteids  of  the  protoplasm  out  of  which 
they  were  evolved.  We  may  have  a  plant  without  chlorojDhyl,  and  a  vege- 
table cell  without  a  cellulose  wall,  but  our  very  conception  of  a  living, 
functionally  active,  cell,  whether  vegetable  or  animal,  is  necessarily  asso- 
ciated with  the  integrity  of  its  protoplasm,  of  which  the  invariable  organic 
constituents  are  proteids. 

"In  the  animal,  the  proteids  claim  even  more  strikingly  our  attention 
than  in  the  vegetable,  in  that  they  form  a  very  much  larger  proportion  of 
the  whole  organism,  and  of  each  of  its  tissues  and  organs.  We  may  indeed 
say  that  the  material  substratum  of  the  animal  organism  is  proteid,  and 
that  it  is  through  the  agency  of  structures  essentially  proteid  in  nature 
that  the  chemical  and  mechanical  processes  of  the  body  are  effected.  It  is 
true  that  the  proteids  are  not  the  only  organic  constituents  of  the  tissues 
and  organs,  and  that  there  are  others,  present  in  minute  quantities,  which 
probably  are  almost  as  widely  distributed,  such  as  for  instance  phosphorus- 
containing  fatty  bodies,  and  glycogen,  yet  avowedly  we  can  (at  the  most) 
only  say  probably,  and  cannot,  in  reference  to  these,  affirm  that  which  we 
may  confidently  affirm  of  the  proteids — that  they  are  indispensable  constit- 
uents of  every  living,  active,  animal  tissue,  and  indissolubly  connected 
with  every  manifestation  of  animal  activity."  (Gamgee,  Physiological 
Chemistry,  Chap.  I.) 

The  molecular  instability  of  proteids  is  proved  bj  tlie  ease 
with  which  they  may  be  decomposed  into  simpler  compounds ; 
their  complex  constitution  by  the  numerous  compounds,  them- 
selves often  highly  complex,  which  may  thus  be  derived  or 
split  off  from  them. 


CARBOHYDRATES  AND  FATS.  37 

Amongst  tlie  otlier  matters  fomul  in  protoplasm  or  closely 
associated  with  it  those  of  most  frequent  occurrence  and  gicatcHt 
physiological  importance  are  two  groups  of  less  complex  sul)- 
stances,  viz.,  carbohydrates  and  fats.  These  contain  carhon,  hy- 
drogen, and  oxygen,  but  no  nitrogen;  they  do  not  ai)j»car  to  he 
closely  related  to  proteids  in  chemical  constitution,  l)ut  they 
occur  to  some  extent  almost  everywhere  in  living  organisms,  and 
in  many  instances  are  known  to  he  of  great  importance,  es])e- 
cially  in  nutrition.  They  are  rich  in  potential  energy  and  mo- 
bile in  molecular  arrangement;  hence  it  is  not  strange  that  tliey 
figure  largely  in  food,  and  are  often  laid  by  as  reserve  ft  m  Mi- 
materials  in  the  organism. 

J^.  Carbohydrates.  These  substances  are  so  called  because, 
besides  carbon,  they  contain  hydrogen  and  oxygen  united  in  the 
same  proportions  as  in  water.  They  include  stai'cli,  various 
kinds  of  sugar,  cellulose,  and  glycogen.  Starch  (C JIjoOJ  is  of 
very  frequent  occurrence  in  plant-cells,  where  it  appears  in  the 
form  of  granules  embedded  in  the  protoplasm  (Fig.  9).  Cel- 
lulose, having  the  same  chemical  formula  as  starch,  but  quite 
different  in  physical  properties,  almost  invariably  forms  the  basis 
of  the  cell-membrane  in  plants. 

C.  Fats.  These  are  of  especial  importance  as  reserves  of 
food-materials  (e.g.,  in  adipose  tissue  and  in  seeds).  They  con- 
tain much  less  oxygen  than  the  carbohydrates;  are  therefore 
more  oxidizable,  and  richer  in  potential  energy.*  They  com- 
monly occur  in  the  form  of  drops  suspended  in  the  protoplasm 
(Fig.  17),  and  are  especially  common  in  animal  cells,  though  by 
no  means  confined  to  them. 

Physical  Relations.  The  appearance,  consistency,  etc.,  of 
protoplasm  have  already  been  described ;  but  it  still  remains  to 
speak  of  certain  of  its  other  physical  properties,  and  especially 
of  the  manner  in  which  its  activity  is  conditioned  by  various 
physical  agents. 

lielations  of  Vital  Action  to  Temperature,  It  is  a  general 
law  that  within  certain  limits  heat  accelerates,  and  cold  dimin- 
ishes, the  activity  of  protoplasm.     We  know  that  cold  tends  to 

*  According  to  careful  researches,  one  pound  of  butter  contains  5654  foot- 
tons,  and  a  pound  of  sugar  2755  foot-tons,  of  energy.  A  pound  of  proteid  is 
nearly  equivalent  in  this  respect  to  a  pound  of  carbohydrate. 


38  PROTOPLASM  AND   THE  CELL. 

benumb  our  own  bodies  (provided  the j  become  really  cliilled),  and  ' 
in  lower  animals  the  heart  beats  more  slowly,  the  movements  be- 
come sluggish  or  cease,  breathing  becomes  slow  and  heavy, — in 
a  word,  all  of  the  vital  actions  become  depressed, — whenever 
the  ordinary  temperature  is  sufficiently  lowered.  If  we  chill 
the  rotating  protoplasm  of  Chara  or  Nitella^  the  vibrating  cilia 
of  ciliated  cells,  or  an  actively  flowing  A^noebay  the  movements 
become  slower,  and  finally  cease  altogetlier. 

On  the  other  hand,  moderate  warmth  favors  protoplasmic 
action.  Benumbed  fingers  become  once  more  nimble  before  the 
warmth  of  the  fire.  In  a  hot  room  the  frog's  heart  beats  more 
rapidly,  cilia  lash  more  energetically,  the  Amoeba  flows  more 
rapidly,  and  the  protoplasm  of  Cliara  courses  more  swiftly.  In 
the  winter  months  the  protoplasm  of  j)lants  and  of  many  animals 
is  in  a  state  of  comparative  inactivity.  Most  plants  lose  their 
leaves  and  stop  growing ;  many  animals  bury  themselves  m  the 
mud  or  in  burrows,  and  pass  the  winter  in  a  deep  sleej)  {Jiiberna' 
tion)^  during  which  the  vital  fires  burn  low  and  seem  well-nigh 
extinguished.  The  warmth  of  spring  re-estabhshes  the  activity 
of  the  protoplasm,  and  in  consequence  animals  awake  from  their 
sleep  and  plants  put  forth  their  leaves. 

But  this  law  is  true  only  within  certain  limits.  Extreme 
heat  and  cold  are  alike  inimical  to  hfe,  and  as  the  temperature 
approaches  these  extremes  all  forms  of  vital  action  gradually  or  ;> 
suddenly  cease.  The  limits  are  so  variable  that  it  is  not  at 
present  possible  to  formulate  any  exact  law  which  shall  include 
all  known  cases.  For  instance,  many  organisms  are  killed  at 
the  freezing-point  of  water  (0°  C);  but  certain  forms  of  life 
have  withstood  a  temperature  of  —  87°  C.  {—  123°  F.),  and  re- 
cent experiments  show  that  frogs  and  rabbits  may  be  chilled  to 
an  unexpected  degree  without  fatal  results. 

The  upper  limit  is  also  inconstant,  though  less  so  than  the  lower. 
Most  organisms  are  destroyed  at  the  temperature  of  boiling 
water  (100°C.),  but  the  spores  of  bacteria  have  been  exposed  to 
a  much  higher  temperature  ^vdthout  destruction  (120"— 125°  C). 
As  a  rule,  protoplasm  is  killed  by  a  temperature  varying  from 
40°  to  50°  C,  the  immediate  cause  of  death  being  aj^parently 
due  to  a  sudden,  coagulation  (p.  36)  of  certain  substances  in  the 
protoplasm.     Thus^   if  a  brainless   frog   be   gradually  heated, 


PROTOPLASM  AND  PHYSICAL  AGENTS.  89 

death  ensues  at  about  -iO^  C,  and  tlie  body  becomes  Ptiff  and 
rigid  [rigor'  caloris)  from  tlie  coagulation  of  tlie  imi.scle-sub- 
stance.  The  lower  forms  of  animal  life  agree  well  with  ])lants 
in  their  "  fatal  temperatures,"  which  in  many  cases  lie  between 
40°  and  50°  C. 

Lastly,  it  appears  to  be  true  that  there  is  a  certain  most 
favorable  or  optimum  temperature  for  the  protoplasm  of  each 
species  of  plant  and  animal,  this  optimum  differing  considera])ly 
in  different  sj^ecies.  Probably  the  highest  limit  occurs  among 
the  birds,  where  the  uniform  temperature  of  the  body  may  be 
as  high  as  40°  C.  The  lowest  occurs  among  the  marine  i)]ant8 
and  animals  of  the  Arctic  seas,  or  of  great  de2)ths,  where  the 
temperature  seldom  rises  more  than  a  degree  or  two  al)ove  the 
freezing-point.  Between  these  limits  there  aj^pears  to  be  great 
variation,  but  35°  C.  may  perhaps  be  taken  as  the  average  op- 
timum. 

Moisture.  Protoplasm  always  contains  a  large  amount  of  water,  of 
which  indeed  the  lifeless  portion  of  living  things  chiefly  consists.  (Se 
table  on  p.  34.)  All  plants  and  animals  are  believed  to  be  killed  by  com- 
plete drying,  though  some  of  the  simpler  forms  resist  partial  drying  for  a 
long  time,  becoming  quiescent  and  reviving  again  when  moistened,  some- 
times even  after  the  lapse  of  years.  Hence  water  appears  to  be  an  essen- 
tial constituent  of  protoplasm,  although,  as  in  the  case  of  mineral  matters, 
we  do  not  know  the  nature  of  its  connection  with  the  other  elements  or 
compounds  present. 

Electricity.  It  has  been  shown  that  many  forms  of  vital  action  are  ac- 
companied by  electrical  disturbances  in  the  protoplasm.  It  is  therefore 
not  surprising  that  the  application  of  electricity  to  living  protoplasm  sliould 
have  a  marked  effect  on  its  actions.  If  the  stimulus  be  very  slight,  proto- 
plasmic movements  are  favored.  Colorless  blood-corpuscles  creep  more 
actively,  and  ciliary  action  increases  in  vigor.  Stronger  shocks  cause  a 
spasmodic  contraction  of  the  protoplasm  (tetanus),  from  which  it  m.-iy  or 
may  not  be  able  to  recover,  according  to  the  strength  of  the  shock. 

Poisons.  Towards  certain  agents  protoplasm  is  indifferent  or  seemingly 
so,  but  towards  others  it  behaves  in  a  very  remarkable  manner.  The  mat- 
ters known  as  poisons  modify  or  destroy  its  activity,  as  is  well  known  from 
the  familiar  effects  of  arsenic,  opium,  etc.  Disease  may  also  interfere  with 
its  normal  activity ;  but  the  consideration  of  these  phases  of  the  subject 
belongs  to  the  more  exclusively  medical  sciences,  such  as  toxicology  and 
pathology. 

Other  Physical  Agents.  The  more  highly  specialized  forms  of  proto- 
plasm are  affected  by  a  great  variety  of  physical  agents,  such  as  light, 


40  ■  PROTOPLASM  AND   THE  CELL. 

sound,  pressure,  etc.,  and  upon  this  susceptibility  depend  many  of  the 
higher  manifestations  of  life.  For  instance,  waves  of  light  or  of  sound, 
acting  upon  special  protoplasmic  structures  in  the  eye  and  ear,  call  forth 
actions  which  ultimately  result  in  the  sensations  of  sight  and  hearing. 
Similar  considerations  apply  to  the  senses  of  smell,  taste,  and  touch  ;  but 
the  discussion  of  all  these  special  modes  of  protoplasmic  action  must  be 
deferred.  Enough  has  been  said  to  show  that  living  organisms  (that  is, 
the  protoplasm  which  is  their  essential  part)  are  able  to  respond  to  many 
influences  proceeding  from  the  world  in  which  they  live.  Upon  this  prop- 
erty depend  the  intimate  relations  between  the  organism  and  its  environ- 
ment, and  the  power  of  adaptability  to  the  environment  which  is  one  of  the 
most  marvellous  and  characteristic  properties  of  living  things. 

Non-cliff iisihility.  Living  protoplasm,  like  most  of  the  various  proteid 
matters  which  it  yields  (p.  36),  is  indffusihle.  It  will  be  seen  eventually 
that  osmotic  processes  play  a  leading  role  in  the  lives  of  plants  and  animals, 
since  they  are  in  large  part  the  means  by  which  nutriment  is  conveyed  to 
the  living  substance.  In  view  of  this  fact,  the  non-diffusibility  of  proto- 
plasm as  well  as  of  ordinary  proteids  is  a  fact  of  much  significance. 

Vegetal  and  Animal  Protoplasm.  The  protoplasm  of  plants  is  es- 
sentially identical  with  that  of  animals  in  chemical  and  physical  relations, 
and  manifests  the  same  fundamental  vital  properties.  But  it  would  mani- 
festly be  absurd  to  suppose  this  identity  absolute,  for  if  it  were  so,  plants 
and  animals  would  also  be  identical ;  and  furthermore,  the  protoplasm 
of  every  species  of  plant  and  animal  must  differ  more  or  less  from  the 
protoplasm  of  every  other  species.  What  is  meant  is  that  the  differences 
between  the  many  kinds  of  protoplasm  are  far  less  important  than  the 
fundamental  resemblances  which  underlie  them. 


CHAPTER  lY. 

THE  BIOLOGY  OF  AN  ANIMAL. 

The  Common  Earthworm. 

{Lumhricus  terrestris,  Linnaeus.) 

We  now  advance  to  a  more  precise  examination  of  the  living 
body  considered  as  an  individual.  It  is  a  familiar  fact  that 
living  things  fall  into  two  great  groups,  known  as  plants  and 
animals.  We  shall  therefore  examine  a  representative  of  each 
of  these  grand  divisions  of  the  living  world,  and  inquire  huw 
they  resemble  each  other  and  how  they  differ.  Any  liigher 
animal  would  serve  as  a  type,  but  the  connnon  earthworm  is  a 
peculiarly  favorable  object  of  study,  because  of  the  sim})licity  of 
its  structure,  the  clearness  of  its  relation  to  other  animals  stand- 
inty  above  and  below  it  in  the  scale  of  orojanization,  and  the  ease 
with  which  it  may  be  procured  and  dissected.  Earthworms,  of 
which  there  are  many  kinds,  are  found  in  all  parts  of  the  world, 
extending  even  to  isolated  oceanic  islands.  In  the  United  States 
there  are  several  species,  of  which  the  most  connnon  are  L. 
commicnis  {Allolobophora  miccosa^  Eisen),  L.  terrestris^  and  Z. 
fmtichis  {Allolopobliora  fmtida^  Eisen).  The  tirst  two  of  these 
are  found  in  the  soil  of  gardens,  etc.,  Z.  terredv'is  l)eing  the 
larger  and  stouter  species  and  readily  distinguishable  by  the 
flattened  shape  of  the  posterior  region,  Z.  fmtldu'^.  a  smaller 
red  species,  transversely  striped,  and  having  a  characteristic 
odor,  occurs  in  and  about  compost-heaps. 

Mode  of  Life,  etc.  Earthworms  live  in  the  earth,  burrow- 
ing through  the  soil  at  a  depth  varying  from  a  few  inches  to 
several  feet.  Here  they  pass  the  daytime,  crawling  out  at 
night  or  after  a  shower.     The  burrows  proceed  at  iirst  straight 

downwards,  and  then  wind  about  irreorularlv,  sometimes  reacli- 

41 


42  THE  BIOLOGY  OF  AN  ANIMAL. 

ing  a  deptli  of  six  or  eight  feet.      The  eartliworm  is  a  nocturnal 
animal,  and  during  the  day  lies  quiet  in  its  burrow  near  the  sur- 
face,   extended   at  full  length,  head  uppermost.     At   night  it 
becomes  very  active,  and,  thrusting  the  fore  end  of  the  body 
far  out,  explores  the  vicinity  in  all  directions,  tliough  still  clinging 
fast,  as  a  rule,  to  the  mouth  of  the  burrow  by  the  hinder  end. 
In  this  way  tlie  worm  is  able  to  forage,  seizing  leaves,  pebbles, 
and   other  small  objects,  and  dragging  them  into  the  burrow. 
Some  of  these  are  devoured ;  the  remainder  (including  the  peb- 
bles, etc.)  are  used  to  hue  the  uj^per  part  of  the  burrow,  and  to 
plug  up  its  opening  when  the  worm  retires  for  the  day.      Be- 
sides bits  of  leaves  and  animal  matter,  earthworms  swallow  large 
quantities  of  earth,  which  is  passed  slowly  through  the  alimentary 
canal,  so  that  any  nutritious  substances  contained  in  it  may  be 
digested  and  absorbed.     This  earth  is  generally  swallowed  at  a 
considerable  distance  below  the  surface  of  the  ground,   and  is 
finally  voided  at  the  surface  near  the  opening  of  tJie  burrow. 
In  this  way  arise  the  small  j^iles  of  earth  ("  castings  "  or  feeces) 
which  every  one  has  seen,  especially  in  the  morning,  wherever 
earthworms  abound.     Yery  large  quantities  of  earth  are  thus 
brought  to  the  surface  by  earthworms — in  some  cases,  accord- 
mg  to  Darwin's  estimates,  more  than  eigliteen  tons  j^er  acre  in 
a  single  year.     In  fact,  most  soils  are  continually  being  w^orked 
over  by  worms;   and   Darwin   has  shown   that   these   humble 
creatures,  in  the  course  of  centuries,  have  helped  to  bury  huge 
rocks  and  the  ruins  of  ancient  buildings.^ 

The  earthworm  has  no  ears,  eyes,  or  any  other  well-marked 
organs  of  special  sense.  Nevertheless — and  this  is  a  point  of 
great  j)hysiological  interest — the  fore  end  of  the  body  is  sensi- 
tive to  light ;  for  if  a  strong  light  be  suddenly  flashed  upon  this 
part  of  the  worm  as  it  lies  stretched  fortli,  it  will  often  ''dash 
like  a  rabbit  into  its  burrow. ' '  The  animal  has  a  keen  sense  of 
touch,  as  may  be  proved  by  tickling  it;  and  its  sense  of  taste 
nmst  be  well  developed,  since  the  worm-  is  somew^hat  fastidious 
in  its  choice  of  food.  Earthworms  appear  to  be  quite  deaf,  but 
possess  a  distinct,  tliough  feeble,  sense  of  smell. 

*  Darwin,  Vegetable  Mould  and  Earthworms.     Appleton,  N.  Y.,  1882.     See 
also  White's  Natural  History  ofSelborne,  Index,  references  to  "  Earthworms.'* 


AN TEIW- POSTERIOR  DIFFEREyilA  TION. 


43 


Gj:neral  Mokpiiology. 

Attention  will  first  Le  directed  to  certain  features  of  tlie 
BODY  seemingly  of  little  importance,  but  really  full  of  meaning 
when  compared  with  like  features  in  other 
animals  higher  or  lower  in  the  scale  of 
organization. 

Antero- posterior  Differentiation.  The 
body  (Fig.  21)  has  an  elongated  cylindrical 
form,  tapering  to  a  hlnnt  point  at  one  end, 
obtusely  rounded  and  fattened  at  the  other. 
As  a  rule,  the  pointed  end  moves  for- 
wards in  locomotion,  and  the  mouth  opens 
near  it.  For  these  and  otlier  reasons 
the  pointed  end  might  be  called  the  head- 
end, and  the  other  tlie  tail-end.  But  the 
worm  has  really  neither  head  nor  tail,  and  SO-y-  —  ' 
hence  the  two  ends  may  better  be  distin- 
guished as  the  fore  end  and  the  hinder  end, 
or  still  better  as  anterior  and  posterior. 
And  in  scientific  language  the  fact  that  the 
worm  has  anterior  and  posterior  ends 
which  differ  from  each  other  is  stated  by 
saying  that  it  shows  antero-posterior  differ- 
entiation. This  simple  fact  acquires  great  ^n 
importance  hi  the  light  of  comparative 
biology;  for  it  may  be  shown  that  the 
antero-posterior  differentiation  of  the  earth- 
worm, insignificant  as  it  seems,  is  only  the 
begining  of  a  series  of  important  modifica- 
tions extending  upwards  through  more  and 
more  complex  stages  to  culminate  in  man 
himself. 

Fio,  21.— Enlarged  view  of  the  anterior  and  posterior 
parts  of  the  body  of  an  earthworm  as  seen  from  the 
ventral  aspect,  ajj,  anus ;  c,  clitellum  ;  g.p.,  glandular 
prominences  on  the  'lWi\  somite  ;  »i,  mouth  ;  <u\,  exter- 
nal openings  of  the  oviducts  ;  p.x.,  prostomium  ;  s,  set^v  ; 
s.r.,  openings  of  the  seminal  receptacles ;  .x.r/.,  external 
openings  of  the  sperm-ducts.  The  form  of  the  body 
varies  greatly  in  life  according  to  the  state  of  expan- 
sion. The  specimen  here  shown  is  from  an  alcoholic 
preparation.     (Slightly  enlarged.)  Q/l 


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44  THE  BIOLOGY  OF  AN  ANIMAL. 

Dorso-ventral  Differentiation.  In  living  or  well-preserved  spe- 
cimens, the  body  is  not  perfectly  cylindrical,  but  is  somewhat 
flattened,  particularly  near  the  posterior  end,  and  has  a  slightly 
prismatic  four-sided  form.  One  of  the  flattened  sides,  slightly 
darker  in  color  than  the  other,  is  habitually  turned  upwards,  and 
is  therefore  called  the  back,  the  opposite  or  lower  side,  commonly 
turned  downwards,  being  the  belly.  For  the  sake  of  accuracy, 
however,  biologists  are  wont  to  speak  of  the  dorsal  asjject  (back) 
and  venti^al  asjyect  (belly)  of  the  body ;  and  the  fact  that  an  animal 
has  a  back  and  belly  differing  from  each  other  in  structure  or 
function,  or  both,  as  in  the  earthworm,  is  expressed  by  saying 
that  the  body  exhibits  dorso-ventral  differentiation.  Tliis,  like 
antero-posterior  differentiation,  is  very  feebly  expressed  in  the 
external  features,  though  clearly  marked  in  tlie  arrangement  of 
the  internal  parts  of  the  earthworm.  In  higher  animals  it 
becomes  one  of  the  most  consjiicuous  features  of  the  body. 

Bilateral  Symmetry.  When  the  body  is  j^laced  in  the  natural 
position,  with  the  ventral  aspect  downwards,  a  vertical  plane 
passing  longitudinally  through  the  middle  will  divide  it  into 
exactly  similar  right  and  left  halves.  Tliis  similarity  is  called 
two-sided  likeness,  or  hilateral  symmetry.  Though  not  very 
obvious  externally,  this  symmetry  characterizes  the  arrangement 
of  all  the  internal  parts ;  and  it  may  be  gradually  traced  up- 
wards in  higher  animals,  until  it  becomes  as  striking  and  perfect 
as  in  the  human  body. 

Thus  a  very  superficial  examination  reveals  in  the  earth- 
worm two  fundamental  laws  of  organization,  viz.,  dfferentia- 
tion  or  the  law  of  difference,  and  symmetry  or  the  law  of  like- 
ness. And  these  laws  are  of  interest  for  the  reason  among 
many  others  that  earthworms,  like  other  organisms,  have  as  a 
race  had  a  history,  have  co7ne  to  he  by  a  gradual  process  (cf. 
J).  99).  And  biology  must  strive  to  answer  the  questions  ho^v  and 
why  certain  parts  have  become  symmetrical  and  others  differ- 
entiated. Without  entering  into  a  full  discussion  of  the  ques- 
tion at  this  point,  it  may  be  said  that  the  main  cause  of  sym- 
metry or  differentiation  has  probably  been  likeness  or  unlikeness 
of  fanction,  or  of  relation  to  the  environment.  Earthworms 
show  antero-posterior  and  dorso-ventral  differentiation,  because 
the  anterior  and  posterior  extremities,  or  the  dorsal  and  ventral 


METAMERISM.  45 

aspects,  have  l)een  differently  used  and  exposed  to  different  con- 
ditions of  environment.  And  on  the  other  liand  the  (jri^anisni  is 
hilaterally  synnnetrical,  because  tlie  two  sides  have  been  similarlv 
used  and  have  been  exposed  to  like  conditions  of  enviroinneiit. 

Metamerism.  Another  general  feature  of  tlie  eartliwoi'in  is 
of  great  importance  in  view  of  the  conditions  existing  in  other 
animals,  including  the  higher  forms.  The  Ijody  is  marked  off 
by  transverse  grooves  into  a  series  of  similar  parts  like  the  joints 
of  a  band)00  lishing-rod,  or  like  the  joints  of  lingers  (Fig.  21). 
These  parts  are  called  inetameres^  or  more  often  somitex,  and 
the  body  is  consequently  said  to  have  a  Qnetameric  structnre,  or 
to  exhibit  metamerism.  From  the  outside,  the  somites  a})]iear  to 
be  ]3i'oduced  simply  by  regular  folds  in  the  skin,  like  the 
wrinkles  between  the  joints  of  our  fingers.  But  as  the  wrinkles 
of  the  lingers  are  only  the  external  expression  of  a  more  funda- 
mental jointed  structure  within,  so  the  external  fohls  sei>arating 
the  somites,  represent  an  internal  division  into  successive  parts, 
which  affects  all  the  organs  of  the  body,  and  is  a  result  of  some 
of  the  most  important  phenomena  of  development. 

The  explanation  of  metamerism  or  ^'■serial  symmetry'  is  one  of  the 
most  difficult  problems  of  morphology.  But  it  will  be  seen  fartlier  on  that 
metamerism,  so  clearly  and  simply  expressed  in  the  earthworm,  can  be 
traced  upward  in  ever- increasing  complexity  to  the  highest  forms  of  life, 
and  suggests  some  of  the  most  interesting  and  fundamental  problems  with 
which  biology — and  especially  morphology — has  to  deal.  Indeed,  the 
comparative  study  of  the  anatomy  of  most  higher  animals  consists  very 
largely  in  tracing  out  the  manifold  transformations  of  their  complicated 
somites,  which  under  many  disguises  can  be  recognized  as  fundamentally 
like  the  simpler  somites  of  the  earthworm. 

Modifications  of  the  Somites.     The  somites  differ  considerably 

« 

in  different  parts  of  the  body.  The  extreme  anterior  end  is 
formed  by  a  smoothly-rounded  knob  called  the  j^rostofniinfi, 
which  is  shown  by  its  mode  of  development  not  to  be  a  true 
somite.  It  forms  a  kind  of  overhanging  upper  lip  to  the  mouthy 
which  lies  just  behind  it  on  the  ventral  aspect.  Behind  the 
mouth  is  the  first  somite,  in  the  form  of  a  ring,"^  interrupted 
above  by  a  backward  2:)rolongation  of  the  prostomium. 

*  In  numbering  the  somites  the  prostomium  must  never  be  reckoned,  the 
first  somite  being  heJiind  the  mouth. 


46 


THE  BIOLOGY  OF  AN  ANIMAL. 


The  somites  from  the  1st  to  the  2Tth  are  rather  broad, 
and  gradually  increase  in  size.  A  variable  number  cf  the 
somites  lying  between  the  7th  and  19th  are  often  swollen  on 
the  ventral  side,  forming  the  so-called  capsulogenous  glands. 
Between  the  2Sth  and  35th  (the  number  and  position  vary- 
ing shghtly  in  different  specimens)  the  somites  are  swollen 
above  and  on  the  sides,  and  the  folds  between  them  arc 
scarcely  defined  except  on  the  ventral  aspect.  Taken  together, 
they  form  a  broad,  conspicuous  girdle  called  the  clitelluiiv 
(Fig.  21,  c),  wdiose  function  is  to  secrete  the  capsule  in  which 
the  eo;2;s  are  laid,  and  also  a  nutritive  milk-like  lluid  for  the  use 
of  the  developing  embryos.  (The  clitellum  is  not  present  in 
immature  specimens.)  Behind  the  clitellum  the  somites  are 
narrower,  somewhat  four-sided  in  cross-section,  and  iiattened 
from  above  downwards.  This  flattening  sometimes  becomes 
very  conspicuous  towards  the  posterior  end.  Towards  the  very 
last  they  decrease  in  size  rather  abruptly,  and  they  end  in  the 
anal  somite,  which  is  perforated  by  a  vertical  slit,  the  amis 
(Fig.  21,  ail).  All  the  somites  are  perforated  by  small  openings 
leading  into  the  interior  of  the  body,  and  forming  the  outlets  of 
numerous  organs ;  the  position  of  these  openings  will  be  de- 
scribed in  treating  of  the  organs.     Each  somite,  excepting  the 

anterior  two  or  three  and  the  last, 
gives  insertion  to  four  groups  of 
short  and  minute  bristles  or  setce., 
wliich  are  arrano^ed  in  four  lono^i- 
tudinal  rows  aloncr  the  bodv.  Two 
of  these  rows  run  along  the  ventral 
aspect,  tw^o  are  more  upon  the 
sides.  The  setse  extend  outwards 
from  the  interior  of  the  bodv, 
where  they  are  supplied  with  small 
muscles  by  which  they  can  be 
turned  somewhat  either  forwards  or  backwards,  and  can  also  be 
protruded  or  ^vithdrawn  (Fig.  22).  The  setsB  are  of  great  use 
in  locomotion.  When  pointed  backwards  they  support  the  worm 
as  it  crawls  forwards ;  when  they  are  turned  forwards  the  worm 
can  creep  backwards.  They  are  of  interest,  therefore,  as  repre- 
senting an  extremely  simple  and  primitive  limb-like  organ.  : 


Fig.  22.— Diagram  to  iUustrate  the 
action  of  the  setae.  The  dotted 
outline  represents  the  position  of 
the  seta  and  its  muscles  when 
bent  in  the  opposite  direction,  m, 
muscles  ;  s,  seta  ;  %i\  body- wall. 


GENERAL  PLAN  OF  TUE  BODY. 


47 


Plan  of  the  Body.  The  Ludy  of  the  earthworm  (Fi(r.  23), 
like  tliat  of  all  higher  aniniaLs,  c(jiisists  of  two  tubes,  one  (al) 
within  the  other  and  separated  from  it  by  a  considerable  space 
or  cavity  {coe).  The  inner  tube  is  the  allment'ir;/  canal,,  open- 
ing in  front  by  tlie  mouth  and  l^ehind  by  the  anuH ',  the  outer 
tube  is  the  body- wall,  and  its  cavity  is  the  hodtj-cavity  or  Ciduni, 


an 


c.vr      ^rf. 


■^^^.m^^ 


0.      c.cL.      n       ^■^^ 


JZ 


Fig.  23. — A,  diagram  of  the  earthworm  as  seen  in  a  longitudinal  section  of  the  body, 
showing  the  two  tubes,  the  coelom,  and  the  dissepiments.  JB,  diagram  of  cross- 
section  :  (7/,  alimentary  tube;  an,  anus;  Cfr,  ccelom;  »i,  mouth.  C,  diagram 
showing  the  arrangement  of  some  of  the  principal  orgaus  :  //i,  moutli ;  an,  anus ; 
al,  alimentary  canal;  ds,  dissepiments;  d.r.,  dorsal  blood-vessel;  r,  ventral  or 
sub-intestinal  vessel ;  c.r.,  circular  vesselb ;  ?j,  nephridia  or  excretary  organs;  <•.(/., 
cerebral  ganglia ;  r.(/.,  ventral  chain  of  ganglia;  r>.(/.,  oviduct;  o.d.,  ovary.  The 
arrows  indicate  the  course  of  the  circulation  of  the  blood. 


The  coelom  is  not,  however,  a  free  continuous  space  extending 
from  end  to  end,  but  is  divided  transversely  by  a  series  of  tliin 
muscular  partitions,  the  dlssejnments,  into  a  series  of  nearlv 
closed  chambers  traversed  by  the  alimentary  canal,  Eacli  (m.iu- 
partment  corresponds  to  one  somite,  the  dissepiments  ])eing 
opposite  the  external  furrows  mentioned  on  p.  45.  All  tlie 
organs  of  the  body  are  originally  developed  from  tlie  walls  of 
these  chambers,  and  some  of  them  (e.g.,  tlie  organs  of  excretion) 
project  into  the  cavities  of  the  cliambers,  that  is  into  the  cadom. 


48  THE  BIOLOGY  OF  AN  ANIMAL, 

In  the  median  dorsal  line  of  eacli  somite  (excepting  the  first 
two  or  three)  is  a  minute  pore  (the  dorsal  j^ore)  which  perfo- 
rates the  body-wall  and  thus  places  the  coelom  in  connection 
with  the  exterior."^  Other  pores  that  pass  through  the  body- 
wall  into  the  cayities  of  yarious  organs  will  be  described  fur- 
ther on. 

Organs  of  the  Animal  Body.  Systems  of  Organs.  The  body  of 
the  earthworm  consists  essentially  of  protoplasm,  and  in  order  that 
so  large  a  mass  of  liying  matter  may  continue  to  exist  and  carry 
on  the  ordinary  life  of  an  earthworm  it  must  be  able  to  obtain 
a  sufficient  supply  of  food;  to  digest  and  absorb  it,  and  dis- 
tribute it  to  all  parts  of  the  body ;  to  build  up  new  protoj^lasm 
and  remoye  waste.  It  must  be  sensitiye  to  external  and  internal 
influences ;  capable  of  motion  and  locomotion.  Aboye  all,  each 
part  must  act  with  reference  to,  and  in  harmony  with,  eyery 
other  part,  so  that  the  organism  may  not  be  merely  an  aggregate 
of  organs,  but  one  body  acting  as  a  unit  or  a  whole. 

T\\Q^Q functions  are  fulfilled  by  the  organs,  respectiyely,  of 

ALIMENTATION,    DIGESTION,    ABSORPTION,    CIRCULATION,    EXCRETION, 

SENSATION,  MOTION,  and  COORDINATION.  All  of  tlicsc  minister  to 
the  welfare  of  the  indiyidual.  The  REPRODucTiyE  function,  on 
the  other  hand,  and  its  corresponding  organs,  serye  to  perpet- 
uate the  species,  thus  ministering  rather  to  the  race  than  to  the 
indiyidual. 

Sets  of  organs  deyoted  to  the  same  function  constitute  syS' 
terns  I  as  the  alimeiitary  system^  the  circulatory  system^  etc. 
Those  wdiich  are  more  immediately  concerned  with  the  income 
and  outgo  of  matter — namely,  the  alimentary,  digestiye,  absorp- 
tiye,  circulatory,  and  excretory  systems — are  sometimes  called  the 
"vegetative  systems  or  systems  of  nutrition '^  while  those  which 
haye  to  do  more  immediately  with  the  relation  of  the  body  to 
its  enyironment,  rather  than  the  indiyidual  itself,  are  called  syS' 
terns  of  relation.  Examples  of  the  latter  are  the  systems  of 
organs  of  support,  motion  (including  locomotion),  sensation,  and 
coordination ;  and  eyen  the  reproductiye  system,  as  relating  chiefly 
to  other  indiyiduals,  finds  a  place  here. 

*  If  living  worms  be  irritated  they  will  often  extrude  a  milky  fluid  from, 
these  pores,  but  the  use  of  the  latter  is  not  well  understood. 


ALIMENTAHY  SYSTEM.  49 

A.    Systems    of    Nutritive    Organs  :     their    Special    Mor- 
puoLOGY  AND  Physiology.      (ForlJsecp.  ()2.) 

Alimentary  System  (Organs  of  Alimentation).  P^artli-wornis 
feed  mainly  upon  leaves  or  decaying  vegetalile  matter,  l»ut 
will  also  eagerly  devour  meat,  fat,  and  other  animal  siih- 
stances.  They  also  swallow  large  quantities  of  earth  from 
which  they  extract  not  only  any  organic  materials  that  it  may 
contain,  but  probably  also  moisture  and  a  small  amount  of  \;ii-i- 
ous  salts.  The  most  essential  and  characteristic  i)ai-t  (jf  their 
food  is  derived  from  vegetal  or  animal  matter  in  the  form  of 
various  organic  compounds,  of  which  the  most  important  are 
jproteids  (protoplasm,  albumen,  etc.),  carhoh  yd  rates  (starch, 
cellulose),  di\\(\.fats.  These  materials  are  used  by  the  animal  in 
the  manufacture  of  new  protoplasm  to  take  the  place  (►f  tliat 
which  has  been  used  up.  It  is,  however,  impossible  for  the  ani- 
mal to  build  these  materials  directly  into  the  substance  (jf  its 
own  body.  They  must  first  undergo  certain  preparatory  chemi- 
cal changes  known  collectively  as  digestion  y  and  only  after  the 
completion  of  this  process  can  all  the  food  be  absorbed  into  the 
cii'culation.  For  this  j^urpose  the  food  is  taken  not  into  the 
body  proper,  but  into  a  kind  of  tubular  chemical  lalioratory 
called  the  alimentary  canal  through  which  it  slowly  passes^ 
being  subjected  meanwhile  to  the  action  of  certain  chemical  sub- 
stances, or  reagents,  known  as  digestive  ferments.  These  sub- 
stances, which  are  dissolved  in  a  watery  liquid  to  form  the  (/iges- 
ti/ve  fluid,  are  secreted  by  the  walls  of  the  alimentary  tube. 
Through  their  action  the  solid  portions  are  liquefied  and  tlie  food 
is  rendered  capable  of  absorption  into  the  proper  body. 

The  alimentary  canal  is  divisible  into  several  ditfcrently  con- 
structed portions  playing  different  parts  in  the  process  of  alimtMi- 
tation.     Going  backwards  from  the  mouth  these  are  as  follows  : 

1.  The  pharynx  (Fig.  24,  jt>/<),  an  elongated  barrel-shapi'd 
pouch  extending  to  about  the  6tli  somite.  Its  walls  are  thick 
and  muscular,  and  from  their  coelomic  surface  numerous  small 
muscles  radiate  on  every  side  to  the  body-wall.  AVhen  these 
muscles  contract,  the  cavity  of  the  pharynx  is  exj^anded ;  and  if 
the  mouth  has  been  previously  applied  to  any  solid  object,  such 
as  a  leaf  or  pebble,  the  pharynx  acts  upon  it  like  a  suction-i>ump. 


50 


THE  BIOLOGY  OF  AN  ANIMAL, 


10. 


15. 


Fig.  24.— Dorsal  view  of  the  anterior  part  of  the  body  of  Lumhricus,  as  it  appears 
when  laid  open  along  the  dorsal  aspect,  ao,  aortic  arch ;  c,  crop ;  eg,  cerebral 
ganglia ;  c.gl,  calciferous  glands ;  d,  dissepiment ;  d.i\  dorsal  vessel ;  g,  gizzard  ; 
CB,  oesopnagns ;  ph,  pharynx ;  ps,  prostomium ;  s.i,  stomach-intestine,  showing 
the  lateral  pouches;  s.r,  seminal  receptacles;  s.v.^,  s.v.^,  s.v.^,  the  three  pairs  of 
lateral  seminal  vesicles. 


ORGANS   OF  ALIMENTATION.  51 

In  tliis  way  the  animal  lays  hold  of  the  various  ol)jects,  nutri- 
tious and  otherwise,  which  it  devours  or  draws  into  its  buri-ow. 

Embedded  in  the  muscular  walls  of  the  pharynx  are  a 
number  of  small  "  salivary  "  glands  of  wliose  function  notliing 
is  definitely  known,  thougli  they  doubtless  2)our  a  digestive  fluid 
into  the  pharyngeal  cavity. 

2.  The  oesophagus  (ce),  a  slender,  thin- walled  tube  extendino" 
from  the  6tli  to  the  loth  somite.  Through  this  the  food  is 
sw^allowed,  being  driven  slowly  along  by  wavelike  (jjeristaltic) 
contractions  (p.  55).  In  the  region  of  the  lltli  and  12t}i 
somites  are  three  pairs  of  small  pouches  opening  at  the  sides  of 
the  oesophagus.  These  are  the  calciferous  glands  {c.gL).  They 
contain  solid  masses  of  calcium  carbonate,  and  Darwin  conjec- 
tures that  their  use  is  partly  to  aid  digestion  by  neutralizing  tlie 
acids  generated  during  the  digestion  of  leaves,  and  i)erhaps 
partly  to  serve  as  an  outlet  for  the  excess  of  lime  in  the  body, 
especially  when  worms  live  in  calcareous  soil. 

3.  The  crop  (<?),  about  the  16th  somite;  a  thin-walled,  sac- 
like dilatation  of  the  alimentary  canal,  which  serves  as  a  reser- 
voir to  receive  the  swallowed  food. 

4.  The  gizzard  (g),  about  the  iTth  somite;  a  cylindrical, 
firm  and  muscular  portion,  lined  by  a  horny  membrane.  In  this 
the  food  is  rolled  about,  squeezed  and  ground  to  prepare  it  for 
digestion  in  the  following  portion,  viz.  : 

5.  The  stomach-intestine  (s.i.),  which  corresponds  physio- 
logically to  both  the  stomach  and  intestine  of  higher  animals. 
This  is  a  straight  thin- walled  tube,  extending  from  the  gizzard 
to  the  anus,  without  convolutions,  not  difi'erentiated  into  stomach 
and  intestine,  and  devoid  of  distinct  glandular  appendages  sui-h 
as  the  liver  or  pancreas  existing  in  the  higher  animals.  The 
digestive  fluid  is  secreted  by  the  w^alls  of  the  alimentary  canal 
itself,  the  surface  of  which  is  much  increased  l)y  the  presence  of 
lateral  pouches  or  diverticula,  one  on  either  side  in  eacli  somite. 
In  front  these  are  large  and  conspicuous,  l)ut  behind  they  gradu- 
ally diminish  in  size  until  scarcely  percei)tible. 

The  inner  surface  of  the  stomach-intestine  is  further  increased  by  a 
deep  inward  fold,  called  the  typJiJosole,  running  longitudinally  along  the 
dorsal  median  line.  The  typhlosole  is  not  visible  on  the  exterior,  but  is 
seen  by  opening  the  stomach-intestine  from  the  side  or  below,  or  upon 


52  THE  BIOLOGY  OF  AN  ANIMAL. 

making  a  cross-section.  It  is  richly  supplied  with  blood- cessels  that  pass 
down  into  its  cavity  from  the  dorsal  vessel  (Fig.  39),  and  its  main  func- 
tion is  probably  to  increase  the  surface  for  the  absorption  of  food  (cf.  the 
"  spiral  valve  "  in  the  intestine  of  sharks.) 

The  outer  surface  of  the  stomach-intestine  is  covered  with  pigmented, 
yellowish-brown  "chloragogue-cells."  These  were  formerly  supposed  to  be 
concerned  with  the  secretion  of  the  digestive  fluid,  and  hence  are  often 
called  "hepatic  cells."  This,  however,  is  probably  an  erroneous  interpreta- 
tion, and  they  are  now  believed  to  be  concerned  with  the  process  of  excre- 
tion (p.  61). 

Digestion.  Digestion  begins  even  before  the  food  is  taken 
into  the  ahnientaiy  canal ;  before  being  swallowed,  the  leaves, 
etc.,  are  moistened  by  digestive  fluid  poured  out  from  the 
mouths  of  the  worms.  The  main  action,  however,  doubtless  goes 
on  in  the  anterior  part  of  the  stomach-intestine  and  diminishes 
as  the  food  passes  backward.  It  has  been  proved  by  experhnent 
that  the  digestive  fluid  acts  on  at  least  two  of  the  three  principal 
varieties  of  organic  food-stuft's,  viz.,  on  proteids  and  on  starch 
(carbohydrate),  and  in  so  far  resembles  the  pancreatic  fluid  of 
higher  animals,  which  it  further  resembles  in  having  an  alkaline 
reaction.  Analogy  leads  us  to  believe  that  the  digestive  fluid 
has  some  action  also  on  fats ;  but  this  has  not  been  proved. 

Krukenberg  and  Fredericq  have  shown  that  the  digestive  fluid  of  the 
earthworm  contains  at  least  three  ferments  ;  and  according  to  the  former 
author  these  occur  only  in  the  stomach-intestine.     They  are  as  follows  : 

1.  Peptic  ferment^  which  has  the  property  in  an  acid  medium  of  con- 
verting proteids  into  soluble  and  diffusible  peptones ;  this  is  therefore 
analogous  to  the  pepsin  of  the  gastric  juice  in  higher  forms. 

2.  TryptiG  ferment^  having  a  similar  action  on  proteids,  but  only  in  an 
alkaline  medium — hence  analogous  to  the  trypsin  of  pancreatic  juice. 

3.  Biastatic  ferment.,  which  converts  starch  into  glucose  (grape-sugar) 
in  an  alkaline  medium — hence  analogous  to  the  ptyalin  of  saliva  and  the 
amylolytic  ferment  of  pancreatic  juice. 

Absorption.  The  ferments  of  the  digestive  fluid  convert  the 
solid  proteids  into  soluble  and  diffusible  peptones,  the  starchy 
matters  into  sugar  (glucose).  These  products  dissolve  in  the 
liquids  present  and  are  then  gradually  absorbed  by  the  walls  of 
the  intestine  as  the  food  passes  along  the  alimentary  canal.  The 
precise  mechanism  of  absorption  is  not  yet  thoroughly  understood, 
but  it  is  probable  that  much  of  the  nutriment  passes  by  diffusion 
(osmosis)  into  the  walls  of  the  stomach-intestine  and  thence  into 


ORGANS  OF  CIRCULATION. 


r)3 


■> 


t^  r\  1^  ^ 


the  blood  for  distribution  to  all  parts  of  the  l)ody.  The  refuse 
remaining  in  the  alimentary  canal  (and  which  has  never  been  a 
part  of  the  body  proper)  is  finally  voided  throuLdi  the  anus  as 
Hastings  ovfcBces.  This  process  of  "  deftecation  "  must  not  be 
confounded  with  that  of  excretion^  which  will  ]>e  desciibed  later. 

Circulatory  System.     The    food,    having    been    absorbed,    is 
distributed  throughout  the  body  by  two  devices. 

1.  Coelomio  Gircxdation.  The  cavity  of  the  ca'lom  is  tilled 
with  a  colorless  fluid  (' '  coelomic  fluid ' ')  which  must  be  regarded  as  a 
kind  of  lymph  or  blood.  By  the  contractions  of  the  body-wall,  as 
the  worm  crawls  about,  the  cadomic  fluid  is  driven  back  and  forth 
through  all  parts  of  the  coelom, 
through  irregular  openings  in  the 
dissepiments.  As  the  digested 
food  is  absorbed  from  the  stomach- 
intestine  a  considerable  part  of  it  is 
believed  to  pass  into  the  coelomic 
fluid,  and  is  thus  conveyed  directly 
to  the  organs  which  this  fluid 
bathes.  The  coelomic  fluid  is  com- 
posed of  two  constituents,  viz.,  a 
colorless  fluid  called  the  plasma, 
and  colorless  isolated  cells  or  coi'- 
jpuscles  which  float  in  the  plasma, 
and  are  remarkable  for  the  fact 
that  tliey  undergo  constant  though 
slow  changes  of  form.  In  fact  they 
closely  resemble  certain  kinds  of 
Ammhce,  and  we  should  certainly 
consider  them  to  be  such  if  we 
found  them  occurring  free  in  stag- 
nant water.  We  know,  however, 
that  they  live  only  in  the  plasiria,  and  have  a  connnon  origin 
with  the  other  cells  of  the  body ;  hence  we  must  regard  them 
not  as  individual  animals,  but  as  constituent  cells  of  the  eartli- 
worm.  The  ccelomic  fluid  is  in  fact  a  kind  of  fisst/f'  consisting 
of  isolated  colorless  cells  floating  in  a  fluid  intercellular  sul)stan('i'. 
These  free  floating  cells  are  probably  the  scavengers  (j>/i(i(/<irt/f*s) 
of  the  body,    devouring  and  destroying  waste   matters.      8onio 


Fig.  25.— Phagocytes,  from  tlie  cnp- 
loinic  fluid  of  the  earthworm.  .1, 
af^glomeration  of  i»lia>:orytes, 
surrounding  a  foreiRn  body;  /?, 
sintrle  uliaLTocyte,  with  vacuole^i. 
(After  MetschnikotT.) 


54  THE  BIOLOGY  OF  AN  ANIMAL. 

suppose  that  they  also  attack  invadmg  parasites  such  as  bacteria. 

2.  Vascular  Circulation.  Besides  the  coelomic  circulation 
there  is  another  and  more  complicated  circulatory  apparatus  con- 
sisting of  branching  tubes,  the  hlood-vessels.,  which  form  a  com- 
plicated system  ramifying  throughout  the  body.  Through  these 
tubes  is  driven  a  red  lluid  analogous  to  the  red  blood  of  higher 
animals,  and  like  it  consisting  oi plasma  and  corpuscles^  the  latter 
being  flattened  and  somewhat  spindle-shaped.  The  red  color  is 
due  to  a  substance,  licmnoglohin^  dissolved  in  the  plasma  and  not  (as 
in  higher  forms)  contained  in  the  corpuscles,  which  are  colorless. 

The  earthworm  is  not  provided  with  a  special  pumping- 
organ  or  heart  for  the  propulsion  of  the  blood,  such  as  we  find 
in  higher  animals.  In  place  of  this  certain  of  the  larger  blood- 
vessels (viz.,  the  "dorsal  vessel"  and  the  ''aortic  arches") 
have  muscular  contractile  walls,  w^hich  propel  the  blood  in  a  con- 
stant direction  by  wave-like  contractions  that  run  along  the 
vessel  from  one  end  to  the  other  ("peristaltic"  contractions,  cf. 
p.  51)  at  regular  intervals  and  thus  give  rise  to  a  "pulse." 
The  contractile  vessels  give  off  other  non-contractile  trunks 
which  divide  and  subdivide  into  tubes  of  extremely  small  calibre 
and  having  very  thin  walls.  The  ultimate  branches,  known  as 
capillaries.,  permeate  nearly  all  the  organs  and  tissues,  in  which 
they  form  a  close  network.  The  stream  of  blood  after  passing 
through  the  capillaries  is  gathered  into  successively  larger  vessels 
which  after  a  longer  or  shorter  course  finally  em23ty  into  the 
original  contractile  trunks  and  complete  the  circuit.  Thus  the 
vascular  system  is  a  closed  system  of  tubes,  and  there  is  reason  to 
believe  that  the  blood  follows  a  perfectly  definite  course,  though 
this  is  not  yet  precisely  determined.* 

We  may  now  consider  the  arrangement  of  the  principal 
tnmks.  The  largest  of  them,  which  is  also  the  most  important 
of  the  contractile  vessels,  is : 

a^  The  dorsal  vessel  (Fig.  24,  cZ.'y.),  a  long  muscular  tube 
lying  upon  the  upper  side  of  the  alimentary  canal.  In  the  liv- 
ing worm  it  may  be  distinctly  seen  through  the  semi-transparent 

*  It  should  be  noted  tbat    in  the  absence  of  a  heart  it  is  difficult  to  distin 
guish  between  "  arteries  "  and  "veins."     We  may  more  conveniently  distin- 
guish "  afferent  vessels,"  carrying  blood  towards  the  capillaries,  and  "  efferent 
vessels,"  carrying  blood  away  from  them. 


BLOOD  -  VESSELS.  55 

skin  as  a  dark-red  band,  wliicli  is  tolerably  strai<^lit  wlii-n  the 
worm  is  extended,  but  is  made  zigzag  hy  contracti(jn  of  the  bod  v. 
If  it  be  closely  observed,  a  sort  of  wavelike  contraction  is  often 
seen  running  from  behind  forwards.  This  may  be  very  clearly 
observed  in  a  worm  stupefied  by  chloroform,  especially  if  it  luw 
been  laid  open  along  the  dorsal  side.  The  dorsal  vessel  then 
ajipears  as  a  deep-red,  somewhat  twisted,  tube  running  along  the 
upper  side  of  the  alimentary  canal.  Wavelike  contractions 
continually  start  from  its  hinder  end  and  run  ra})i(Ily  forwards, 
one  after  another,  to  the  anterior  end,  where  the  dorsal  vessel 
finally  breaks  up  on  the  pharynx  into  a  large  number  of  branches 
(Fig.  24). 

The  result  of  these  orderly  progressive  contractions  is  that 
the  fluid  within  the  tube  is  pushed  forwards — very  much  as  the 
fluid  in  a  rubber  tube  is  forced  along  when  the  tube  is  strip})ed 
through  the  fingers.  It  is  still  better  illustrated  by  the  action 
of  the  fingers  in  the  operation  of  milking.  This  action  of  the 
vessels  is  a  typical  example  oi  jperistaltic  contraction, 

h.  Suh -intestinal  vessel.  This  is  a  straight  vessel  which 
runs  along  the  middle  line  on  the  lower  side  of  the  alimentary 
canal,  parallel  to  the  one  just  described.  It  returns  to  the 
hinder  part  of  the  body  the  fluid  which  has  been  carried 
forwards  by  the  dorsal  vessel.  On  the  pharynx  it  breaks  up 
into  many  branches,  which  receive  the  fluid  from  corresponding 
branches  of  the  dorsal  vessel. 

c.  Circular  or  comtnissural  vessels.,  metamerically  repeated 
trunks  which  run  from  the  dorsal  vessel  downwards  around  the 
alimentary  canal  and  ultimately  connect  with  the  ventral  vessel. 
They  are  of  several  kinds,  of  which  the  most  important  are  as 
follows : 

1.  The  aortic  arches  or  circunfioasojphageal  vessels,  often 
known  as  "hearts,"  since  like  the  dorsal  vessel  they  are  con- 
tractile and  with  the  latter  furnish  the  entire  propulsive  forco 
for  the  circulation.  These  are  five  pairs  of  large  vessels  en 
circuling  the  oesophagus  in  somites  7  to  11  inclusive.  Tlies*» 
vessels  pass  directly  from  the  dorsal  to  the  ventral  vessel,  giving 
off  no  branches.  During  life  they  perform  powerful  peristakio 
contractions,  receiving  blood  from  the  dorsal  vessel  and  pumping 
it  into  the  sub-intestinal  or  ventral. 


5Q  THE  BIOLOGY  OF  AN  ANIMAL. 

2.  Dorso-intestinal  vessels^  passing  from  the  dorsal  vessel 
into  the  wall  of  the  gut  in  the  region  of  the  stomach-intestine. 
Of  these  vessels  there  are  two  or  three  pairs  in  each  somite. 
Thej  are  thickly  covered  (like  the  dorsal  vessel  in  this  region) 
with  pigmented  ' '  chloragogue-cells, ' '  so  that  their  red  color  is 
usually  not  apparent.  Unlike  the  aortic  arches  these  vessels 
break  up  on  the  wall  of  the  intestine  into  capillaries  which  are 
continuous  with  branches  from  the  ventral  vessel. 

3.  Dorso-teguTnentary  vessels^  passing  from  the  dorsal  vessel 
along  the  dissepiment  into  the  body -wall  on  each  side.  Tliese 
are  small  vessels  that  pass  directly  around  the  body  to  connect 
with  a  longitudinal  trunk  ("  sub-neural  ")  lying  below  the  ven- 
tral nerve-cord  (see  below),  and  giving  off  branches  to  the  body- 
wall,  dissepiments,  and  nephridia. 

Course  of  the  Blood.  The  precise  course  of  the  blood  in 
Lumbricus  is  still  in  dispute,  though  its  more  general  features  are 
known.  It  is  certain  that  the  bulk  of  the  blood  passes  forward  in 
the  dorsal  vessel,  downward  around  the  gut  through  the  aortic  arches 
into  the  ventral  vessel,  and  thence  backwards  towards  the  pos- 
terior region.  Its  path  thence  into  the  dorsal  vessel  is  doubtful. 
The  most  probable  view  is  that  the  blood  proceeds  from  the  ven- 
tral vessel  through  ventro-intestinal  vessels  to  the  capillaries  of 
the  intestine  and  thence  to  the  dorsal  vessel  throuD^h  the  dorso- 
intestinal  vessels.  It  is  possible,  however,  that  the  return  path 
is  through  the  dorso-tegumentary  vessels  and  that  the  dorso- 
intestinal  carry  blood  y;"c>m  the  dorsal  vessel  to  the  intestine. 

In  the  foregoing  account  only  the  more  obvious  features  of  the  blood- 
vessels have  been  mentioned,  and  many  important  details  have  been  passed 
over.  The  circular  vessels  of  the  stomach-intestine  can  be  followed  for 
only  a  short  distance  out  from  the  dorsal  vessel,  v\rhere  they  seem  to  break 
up  into  a  large  number  of  small  parallel  vessels  lying  close  together  and 
running  around  to  the  lower  side.  The  efferent  vessels  do  not  directly  join 
the  sub-intestinal,  but  empty  into  a  sinus  or  vessel  which  runs  parallel  to 
tne  latter,  closely  imbedded  in  the  wall  of  the  stomach-intestine.  The  sub-- 
intestinal  vessel  proper  is  quite  separate  from  the  stomach-intestine,  and 
communicates  by  short  branches  (usually  two  in  each  somite)  with  the 
vessel  lying  above  it.  This  maybe  clearly  seen  in  the  region  of  the  gizzard. 
On  this  there  is  a  variable  number  of  small  lateral  vessels,  which  break  up 
partly  into  a  branching  network,  and  are  partly  resolved  into  extremely 
fine  parallel  vessels  surrounding  the  organ.  On  the  crop  are  three  or  four 
pairs  of  lateral  branches  from  the  dorsal  vessel  which  branch  out  into  a 


BLOOD-VESSELS. 


57 


fine  network,  but  do  not  break  up  into  parallel  vessels  as  o\\  the  gizzard. 
In  the  two  somites  (13th  and  14th)  in  front  of  the  crop  there  are  usually 
two  pairs  of  vessels  running  around  the  cesophagiis.  In  the  11th  and  12th 
somites  a  small  branch  is  given  otf  to  each  calciferous  gland.  The  most 
anterior  pair  of  circular  vessels  are  in  the  6th  somite,  and  are  very  small. 
In  front  of  this  the  dorsal  vessel  breaks  up  into  the  pharyngeal  network. 
In  front  of  the  11th  somite  there  are  three  sub-iutestinal  vessels.  The  two 
additional  vessels  lie,  one  on  either  side  of  the  primary  one  and  break  up 
into  branches  at  the  sides  of  the  pharynx.  The  aortic  arches  empty  into 
the  middle  vessel,  and  at  the  point  of  junction  there  is  a  communication 
with  the  lateral  vessel  of  the  corresponding  side. 

Besides  the  dorsal  and  sub-intestinal  vessels  there  are  three  other  minor 
longitudinal  trunks  (Fig.  26).     Two  of  these  are  very  small,  and  lie  on 


56.  n.. 


Fig.  26.— Dorsal  view  of  part  of  the  ventral  nerve-cord,  showing  the  arranpromeni  of 
the  vessels  of  the  ventral  region,  d.%  dissepiment;  si,  sub-intestinal  dp  ventral 
blood-vessel ;  s?>./?.,  siib-neural ;  sp.??.,  supra-neural.  The  suli-intcstinal  recrivrs  on 
either  side  the  ventro-laterals  (r./)  from  the  nepliridia,  of  which  it  forms  th«'  ef- 
ferent vessel  (e./).  The  sub-neural  is  joined  on  each  side  by  a  continuation  of  the 
d<yrso-tegumentary  (d.t.);  a/,  afferent  branch  to  the  nephridiura  (cf.  Fig.  27). 

either  side  above  the  nerve-cord  (p.  66),  sending  fine  branches  out  from 
each  ganglion  along  the  lateral  nerves.  These  are  the  a  up  ra- neural  trunks 
(.9.W.).  The  third  longitudinal  vessel  {suh-neural)  lies  below  the  nerve-cord. 
(See  Fig.  26.)  It  receives  on  each  side  the  termination  of  the  dorso-tegii- 
mentary  vessel  {d.t.,  Fig.  26)  which  in  its  course  is  connected  witii  the 
capillary  networks  of  the  body-wall  and  the  dissej)iment,  and  gives  off  a 
large  branch  to  the  nephridium  (cf.  Fig.  27). 


58 


THE  BIOLOGY  OF  AN  ANIMAL. 


Besides  the  lateral  vessels  from  the  sub-neural  and  supra-neural  a  pair 
of  "  ventro-lateral "  (y.Z.,  Figs.  26  and  27)  are  given  off  in  each  somite  from 
the  sub-intestinal  to  the  nephridium,  probably  receiving  from  it  the  blood 
Rrhich  originally  entered  through  a  branch  of  the  dorso-tegumentary. 


ST  2/:  7 


im 


JTITV. 


Fig.  27.— Nephridia  of  JjumbiHcns.  A  showing  the  regions  of  the  tube,  B  the  vascular 
supply.    /,  II,  III,  the  three  principal  loops. 

.4.  /,  funnel;  n.t,  the  "narrow  tube";  m.t,  middle  tube;  u\t,  wide  tube:  m.p,  mus- 
cular tube  or  end-vesicle  ;  ds,  dissepiment.  The  narrow  tube  extends  from  a  to  g 
and  is  ciliated  between  a  and  ft,  at  c,  and  from  d  to  c.  The  middle  (ciliated)  tube 
extends  from  g  to  h,  the  wide  tube  from  h  to  k,  where  it  opens  into  the  muscular 
part ;  eX,  external  opening. 

B,  Letters  as  before ;  d.t,  dorso-tegumentary  vessel,  bringing  blood  from  the  dorsal 
vessel,  receiving  at  s  a  branch  from  the  body- wall,  sending  an  afferent  branch  to 
the  nephridium,  and  finally  joining  the  sub-neural  (s.n);  v.l,  ventro-lateral  vessel 
carrying  the  blood  from  the  nephridium  to  the  sub-intestinal  or  ventral  vessel 
(s.i) ;  v.v,  ventral  nerve-cord.  (After  Benham;  the  direction  of  the  blood-cur- 
rents according  to  Bourne.) 

Excretory  System.     It  is  the  office  of  the  excretory  system  to 
remove  from  the  body  proper  the  waste  matters  ulthnately  re- 


ORGANS  OF  EXCRETION.     NEPllRIDIA. 


m 


fi'J'.: 


suiting  from  the  breaking  down  of  living  tissue.  Tliis  docs  not 
mean  the  passing  away  of  the  refuse  of  digestion  throiigli  tlie 
anus  (defaecation,  p.  53),  for  sueh  matters  have  never  been 
absoi'bed  and  therefore  have  never  really  been  witliin  tlic  IkkIv 
proper.  Excretion  means  the  removal  from  the  body  of  matter 
which  has  really  formed  a  part  of  its  substance,  l)ut  lias  been 
used  up  and  is  no  longer  alive.  In  higher  animals  this  function 
is  performed  chieily  by  the  kidneys,  the  lungs,  and  the  skin,  the 
waste  matters  passing  off  in  the  urine,  the  breath,  and  the  sweat. 
In  the  earthworm  it  is  principally  performed  by  small  organs 
called  nephridia^  of  which  here  are  two  in  each  somite,  excej)t- 
ing  the  lirst  three  or  four  (Fig.  29). 

Each   nepln-idium   (Fig.   27)   consists   of  a  long  convoluted 
tube,    attached  to  the  hinder  face  of  a 
•dissepiment,  and  lying  in  the  coilom  at 
the  side  of  the  alimentary  canal.      At 
one    end   the  tube   passes   through   the   ^>^:/;}::;'lii^v;;:-  .; 
body -wall  and  opens  to  the  exterior  by  a  ^^B^;^:^^^ 
minute  pore  situated  between  the  outer        *  '  "**" 
and    inner   rows  of  setie  (p.  46).     The 
other  end  of  the  tube  passes  through  the 
dissepiment    very    near    to    the    point 
where  this  is  penetrated  by  the  nerve - 
cord    (p.   ^^)^    and    opens    by  a  broad, 
funnel-like  expansion  into  the  cavity  of  ^^^  ^^  _.^  nephridiai  funnel 

the    next    somite    in    front    (/",    Fig.    27).       much  enlarKed,  showing  the 
rr^i  •  J*   j^i       J"  1  1   j_i       •  cilia,  the  betfinnin^;  of   tl>o 

The  margms  of  the  funnel  and  the  nmer     ^.^^^,^^^^^  ^,^,^^^  (,)^  ,,,,j  the 
surface  of  the  upper  part  of  the  tube  are    o^ter  sheath  («). 
densely  covered  with  powerful  cilia  (Fig.  28),  whose  action  tend.«=i 
to  produce  a  current  setting  from  the  coelom  into  the  fuimel  and 
through  the  nephridium  to  the  exterior. 

The  coils  of  the  nephridium  arc  disposed  in  three  principal  loops  (I,  II, 
III  in  Fig.  27).  The  tube  itself  comprises  five  very  distinct  regions,  as 
follows  : 

1.  1\\Q  funnel  ov  nej^hrostome ;  much  flattened  from  above  downwards, 
with  the  opening  reduced  to  a  horizontal  chink.  It  is  composed  of  beau- 
tiful ciliated  cells  set  like  fan-rays  around  its  edge.     It  leads  into 

2.  The  "  narrow  tube'''  {n.t.),  a  very  delicate  thin-walled  contorted  tubo 
extending  from  the  nephrostome  through  the  first  loop  and  a  part  of  the 
second.     In  certain  parts  of  its  course  (a  to  6,  at  c,  and  from  d  to  e)  this 


60  THE  BIOLOG  Y  OF  AN  ANIMAL. 

■* 

tube  contains  cilia  which  are  arranged  in  two  longitudinal  bands  on  the 
inner  surface.     At  g  it  passes  into  the 

3.  "  Middle  tube''''  {m.t.)  (g  toh),  extending  straight  through  the  second 
loop,  of  greater  diameter,  ciliated  throughout,  and  with  pigmented  walls. 
At  h  it  opens  into  the 

4.  "  Wide  tube''''  (w.t.).  This  is  of  still  greater  calibre,  with  granular 
glandular  walls  and  without  cilia.  It  extends  through  the  second  loop 
(from  h  to  i,  II)  into  and  through  the  first  from  i  to  J,  and  finally  into  the 
third,  opening  at  k  into  the 

5.  Muscular  part  or  duct  (m.p.)  which  forms  the  third  loop  and  opens  to- 
the  exterior  at  ex.  This,  the  widest  part  of  the  entire  nephridium,  has 
muscular  w^alls  and  forms  a  kind  of  sac  or  reservoir  like  a  bladder,  in 
which  the  excreted  matter  may  accumulate  and  from  which  it  may  be 
passed  out  to  the  exterior. 

The  various  parts  of  the  nephridium  are  held  together  Vjy  connective 
tissue  (p.  90),  and  are  covered  with  a  rich  network  of  blood-vessels,  the 
arrangement  of  which  is  shown  in  Fig.  27,  B.  The  smaller  vessels  usually 
show  numerous  pouchlike  dilatations  which  must  serve  to  retard  the  flow 
of  blood  somewhat.  The  vessels  supplying  the  nephridium  are  connected 
(Fig.  27,  B)  on  the  one  hand  with  the  sub-intestinal  vessel  through  the 
ventro-lateral  trunks  {v.  I.) ;  on  the  other  hand  with  the  sub-neural  (s.n.)  and 
dorsal  vessels,  through  the  dorso-tegumentary  (d.t.).  The  course  of  the 
blood  is  somewhat  doubtful.  According  to  the  view  here  adopted  (cf.  p.  56) 
the  blood  proceeds  from  the  dorso-tegumentary  trunk  to  the  nephridia  and 
thence  through  the  ventro  lateral  to  the  sub-intestinal,  as  shown  by  the 
arrows  in  the  figure.  Benham  (from  whom  the  figures  are  copied)  adopts, 
the  reverse  view.  The  development  of  the  nephridium  shows  that  its 
ciliated  and  glandular  portions  arise  from  a  solid  cord  of  disk-shaped  cells 
which  afterwards  becomes  tubular  by  the  hollowing  out  of  its  axial  portion. 
The  tube  is  therefore  comparable  to  a  drain-pipe  in  which  each  cylinder 
represents  a  cell.  Its  cavity  is  not  intercellular  (between  the  cells,  like  the 
alimentary  cavity),  but  intracellular  {vMliin  the  cells,  like  a  vacuole). 

The  mode  of  action  of  the  nephridia  is  as  yet  only  partially 
understood,  though  there  is  no  doubt  regarding  their  general  char- 
acter. It  is  certain  that  their  principal  office  is  to  remove  from 
the  body  waste  nitrogenous  matters  resulting  from  the  decompo- 
sition of  proteids ;  and  there  is  reason  to  believe  that  these  waste 
matters  are  passed  out  either  as  urea  (  [NHJjCO)  or  as  a  nearly 
related  substance,  together  with  a  certain  quantity  of  water  and 
inorganic  salts. 

Excretion  in  Lumbricus  appears,  however,  to  involve  two  quite  distinct 
actions  on  the  part  of  the  nephridia.  In  the  first  place  the  glandular  walls 
of  the  tube,  which  are  richly  supplied  with  blood-vessels,  elaborate  certain 
liquid  waste  substances  from  the  blood  and  pass  them  into  the  cavity  of 


BBEATUING.  61 

the  tube.  In  the  second  place  the  ciliated  funnels  are  believed  to  take  up 
solid  waste  particles  floating  in  the  ca'lomic  fluid  and  to  pass  tiiem  on  into 
the  tube,  whence  they  are  ultimately  voided  to  the  exterior  together  with 
the  liquid  products  described  above.  It  is  nearly  certain  that  these  parti- 
cles are  derived  from  the  breakii^.g  up  of  "  lynipiioid  "  cells,  some  of  wliicU 
may  have  been  phagocytes  (p.  53;,  floating  in  the  coelomic  fluid,  and  that 
most  if  not  all  of  these  cells  arise  from  ''  chloragogue  cells  "  set  free  from 
the  surface  of  the  blood-vessels  and  of  the  intestine. 

Respiration.  Respiration,  or  breathing,  is  a  twofold  oj^eration, 
consisting  of  the  taking  in  of  free  oxygen  and  tlie  giving  oil  of 
carbon  dioxide  by  gaseous  diffnsion  tlirougli  the  surface  of  tlie 
body.  Strictly  speaking,  this  free  oxygen  must  Ije  regarded  nA 
food,  while  carbon  dioxide  is  to  be  regarded  as  one  of  the  excre- 
tions. Hence  respiration  is  tril)utary  both  to  alimentation  and  to 
excretion ;  but  since  many  animals  possess  special  mechanisms  to 
carry  on  respiration,  it  is  convenient  and  customary  to  treat  of 
it  as  a  distinct  process. 

Respiration  is  essentially  an  exchange  of  gases  between  tlie 
blood  and  the  air,  carried  on  through  a  delicate  membrane  lying 
between  them.  The  earthworm  represents  the  sim})lest  condi- 
tions possible,  since  the  exchange  takes  place  all  over  the  body, 
precisely  as  in  a  plant.  Its  moist  and  delicate  walls  are  every- 
where traversed  bv  a  fine  network  of  blood-vessels  Ivinii:  lust 
beneath  the  surface.  The  oxvffen  of  the  air,  either  in  the 
atmosphere  or  dissolved  in  w^ater,  readily  diffuses  into  the  blood 
at  all  points,  and  carbon  dioxide  makes  its  exit  in  the  reverse 
direction.  Freed  of  carbon  dioxide  and  enriched  with  oxygen, 
the  blood  is  then  carried  away  by  the  circulation  to  the  inniT 
parts,  where  it  gives  up  its  oxygen  to  tlie  tissues  and  becomes 
once  more  laden  with  carbon  dioxide. 

In  higher  animals  it  has  been  proved  that  the  red  cc>loring 
matter  (haemoglobin)  is  the  especial  veliicle  for  the  al)sorptioii 
and  carriage  of  the  oxygen  of  tlie  blood,  entering  into  a  looso 
chemical  union  with  it  and  readily  setting  it  free  again  under  the 
appropriate  conditions.  This  is  doubtless  true  in  the  earthworm 
also. 

It  is  interesting  to  study  the  various  devices  by  which  this  function  is 
performed  in  different  animals.  In  the  earthworm  the  whole  outer  surface 
is  respiratory,  and  no  special  respiratory  organs  exist.  In  other  animals 
such  organs  arise  simply  by  the  differentiation  of  certain  regions  of  the 


62  THE  BIOLOGY  OF  AN  ANIMAL. 

general  surface,  which  then  carry  on  the  gaseous  exchange  for  the  whole 
organism.  In  many  aquatic  animals  such  regions  bear  filaments  or  flat 
plates  or  feathery  processes  known  as  gills  or  hrancluce^  which  are  bathed 
by  the  water  containing  dissolved  air,  though  in  many  such  animals 
respiration  takes  place  to  some  extent  over  the  general  surface  as  well.  In 
insects  the  respiratory  surface  is  confined  to  narrow  tubes  {trachece)  which 
grow  into  the  body  from  the  surface  and  branch  through  every  part,  but 
must  nevertheless  be  regarded  as  an  infolded  part  of  the  outer  surface. 
In  man  and  other  air-breathing  vertebrates  the  respiratory  surface  is 
mainly  confined  to  the  lungs,  which  are  simply  localized  infoldings  of  the 
•outer  surface  specially  adapted  to  effect  a  rapid  exchange  of  gases  between 
the  blood  and  the  air. 

It  is  easy  to  see  why  special  regions  of  the  outer  surface  have  in  higher 
animals  been  set  aside  for  respiration.  It  is  essential  to  rapid  difi'usion 
that  the  respiratory  surface  should  be  covered  with  a  thin,  moist  membrane, 
and  it  is  no  less  essential  that  many  animals  should  be  provided  with  a 
firm  outer  covering  as  a  protection  against  mechanical  injury  or  desicca- 
tion. Hence  the  outer  surface  becomes  more  less  distinctly  differentiated 
into  two  parts,  viz.,  a  protecting  part,  the  general  integument ;  and  a 
respiratory  part,  which  is  usually  preserved  from  injury  by  being  folded 
into  the  interior  as  in  the  case  of  lungs  or  tracheae,  or  by  being  covered 
with  folds  of  skin  as  in  the  gills  of  fishes,  lobsters,  etc.  This  covering  or 
turning  in  of  the  respiratory  surfaces  brings  with  it  the  need  of  mechanical 
arrangements  for  pumping  air  or  water  into  the  respiratory  chamber  ;  and 
thus  arise  many  complicated  accessory  respiratory  mechanisms. 

B.     Orgaxs  of  Relation.      (For  A  see  p.  49.) 

Motor  System.  The  movements  of  the  body  have  a  twofold 
purpose.  In  the  first  place  they  enable  the  animal  to  alter  its 
relation  to  the  environment,  to  move  about  (locoinotion)^  to  seize 
and  swallow  food,  and  to  perform  various  adaptive  actions  in 
response  to  changes  in  the  environment.  In  the  second  place, 
the  movements  may  alter  the  relation  of  the  various  parts  of  the 
body  one  to  another  {visceral  movements  and  the  like),  such  as 
the  movements  which  propel  the  blood,  drive  the  food  along  the 
alimentary  canal  and  roll  it  about  (p.  49),  those  which  ex|3el 
waste  matters  from  the  nephridia,  discharge  the  reproductive 
products,  etc. 

Most  of  these  movements  are  performed  by  structures  known 
as  m^uscles^  which  consist  of  elongated  cells  (fibres)  endowed  in  a 
high  degree  with  the  power  of  contractility — i.e.,  of  shortening, 
or  dra^ving  together  (cf.  p.  27).     Ordinary  ''muscles"  are  in 


MUSCLES.  63 

tlie  form  of  long  bands  or  sheets  of  parallel  Hbres,  such  as  those 
that  form  the  body-wall,  that  move  the  setse,  and  dilate  tlic 
pharynx.  Other  muscular  structures,  liowever,  do  not  form  dis- 
tinct "  nmscles,"  but  consist  of  muscular  fi])rcs  more  or  less 
irregularly  arranged  and  often  intermingled  with  otlier  kinds  of 
tissue.  Of  this  character  are  the  nniscular  walls  of  tlie  contrac- 
tile vessels,  and  of  the  muscular  portions  of  the  nepliridia  and 
dissepiments.  It  is  clear  from  the  above  that  the  nuiscular  sys- 
tem is  not  isolated,  but  is  intimately  involved  in  many  organs. 

The  muscles  of  the  body-wall  are  arranged  in  two  concentric  layers 
below  the  skin.  In  the  outer  layer  the  muscles  run  around  the  body,  and 
are  therefore  called  circular  muscles.  Those  of  the  inner  layers  have  a 
longitudinal  course, — i.e.,  parallel  with  the  long  axis  of  the  body, — and 
are  arranged  in  a  number  of  different  bands.  The  most  important  of  these 
are : 

1.  The  dorsal  hands  (Fig.  39),  one  on  either  side  above,  in  contact  at 
the  median  dorsal  line,  and  extending  down  on  either  side  as  far  as  the 
outer  row  of  setae. 

2.  The  ventral  hands,  on  either  side  the  middle  ventral  line  and  occupy- 
ing the  space  between  the  two  inner  (lower)  rows  of  setaB. 

3.  The  lateral  hands,  occupying  the  space  on  either  side  between  tlie 
two  rows  of  setae. 

All  these  vary  greatly  in  different  regions  of  the  body,  and  in  some  parts 
become  more  or  less  broken  up  into  subsidiary  bands.  There  is  also  a 
narrow  band  traversing  the  space  between  the  two  setae  of  each  group. 

The  setce,  which  may  be  reckoned  as  part  of  the  motor  system,  are  pro- 
duced by  glandular  cells  covering  their  inner  ends,  and  tliey  grow  con- 
stantly from  this  point,  somewhat  as  hairs  grow  from  the  root.  After 
being  fully  formed,  and  after  a  certain  amount  of  use,  the  setic  are  cast 
off  and  replaced  by  new  ones  which  have  meanwhile  been  forming.  lu 
each  group  we  find,  therefore,  setae  of  different  sizes.  At  their  inner  ends 
they  are  covered  by  a  common  investment  of  glandular  cells  wliich  appears 
as  a  slight  rounded  prominence  when  viewed  from  within.  These  prom- 
inences are  called  the  setigeroiis  glands.  When  a  worm  is  laid  open  from 
above,  the  glands  are  seen  in  four  parallel  rows,  two  of  wliich  lie  on  either 
side  of  the  nerve-cord  (see  Fig.  29). 

Each  group  of  setae  is  provided  with  special  retractor  or  protractor 
muscles,  and  a  narrow  muscular  band  passes  from  the  upper  to  tlie  lower 
group  on  each  side  internal  to  the  body-wall. 

Cilia.  A  second  set  of  motor  organs  are  cilia  (their  mode  of  action  has 
been  referred  to  on  p.  31),  which  are  of  the  utmost  importance  m  the 
life  of  the  earthworm.  They  cover  the  inner  surface  of  the  stomach-intes- 
tine (where  they  doubtless  assist  in  the  movements  of  the  food)  play  the 
important  part  in  excretion  already  described,  collect  and  help  to  discharge 


64  THE  BIOLOGY  OF  AN  ANIMAL. 

the  reproductive  elements  (p.  74),  and  assist  in  the  fertilization  of  the  Qg^ 
(p.  74).  Their  action,  like  that  of  the  muscle-fibres,  is  doubtless  due  to  the 
property  of  contractility.,  the  protoplasm  alternately  contracting  on  opposite 
sides  of  the  cilium  and  thus  causing  its  whiplike  action. 

White  Blood-corpuscles.  Amoeboid  Cells.  Lymph-cells.  Phagocytes, 
Besides  muscle-cells  and  ciliated  cells  there  is  a  third  variety  which  display 
contractility  and  movement.  These  are  the  coelomic  corpuscles  referred  to 
above  (p.  53).  Until  recently  their  function  was  wholly  unknown,  but  it 
is  now  generally  believed  that  they  are  the  scavengers  of  the  body,  devour- 
ing the  dead  tissues  or  foreign  bodies  which  invade  the  organism.  Whether 
they  also  attack  and  devour  living  parasites  such  as  Qregarina  and  Bacteria 
is  not  yet  fully  determined.  They  move  their  parts  much  as  Amoebae  do, 
engulfing  particles  about  them  by  a  kind  of  flux. 

Nervous  System.     Organs  of  Coordination. 

Introduction.  The  general  office  of  the  nervous  system  of 
organs  is  to  regnlate  and  coordinate  the  actions  of  all  the  other 
parts  in  such  wise  that  these  actions  shall  form  an  harmonious 
and  orderly  whole.  Through  ner\'ous  organs  the  worm  receives 
from  the  environment  impressions  which  pass  inwards  through 
the  nerves  as  sensory  or  afferent  unpulses,  to  the  nervous  centres ; 
and  through  other  nervous  organs  impulses  (efferent  or  motor) 
pass  outwards  from  the  centres  to  the  various  parts  so  as  to 
arouse,  modify,  or  suspend  their  activities.  Tlius  tlie  animal  is 
enabled  to  call  forth  movements  resulting  in  the  two  kinds  of 
adjustments  referred  to  on  p.  6i^,  viz.,  {a)  adjustments  of  the 
body  as  a  whole  to  changes  in  the  environment  (e.  g. ,  the  with- 
drawal of  the  earthworm  into  its  burrow  at  the  approach  of  day) ; 
and  ih)  adjustments  between  the  parts  of  the  body  itself,  so  that 
a  change  in  one  part  may  call  forth  answering  changes  in  other 
parts  (e.g.,  the  increased  supply  of  blood  to  tlie  alimentary  canal 
during  digestion,  or  vigorous  movements  of  the  fore  end  of  the 
body  when  the  hind  end  is  irritated). 

These  functions  are  always  performed  by  one  or  more  nerve- 
cells^  which  give  off  long  slender  branches  known  as  nerve-fibres 
usually  gathered  together  in  bundles,  the  nerves^  extending  into 
all  parts  of  the  body.  In  all  higher  animals  the  main  bulk  of 
the  nerve- cells  are  aggregated  in  definite  bodies  known  as 
ganglia.^  out  of  which,  into  which,  or  through  which,  the  nerves 
proceed ;  and  as  a  matter  of  convenience  it  is  customary  to  desig- 
nate the  most  important  of  these  ganglia  collectively  as  the  cen- 


NERVES  AND  GANGLIA.  i5f) 

tral  nervous  system.  The  remaining  p(jrti«jn,  which  consists 
mainly  of  nerve-fibres,  tliongli  it  may  also  contain  many  nerve- 
cells  and  small  sporadic  ganglia,  is  known  as  the  peripheral 
Tiervoiis  system. 

Gener'ol  Anatomy  of  the  Nervous  System.  In  the  earth- 
v^^orm  the  central  system  consists  of  a  long  series  of  douhk*  gangUa, 
metamerically  repeated,  and  connected  by  nL;rvL*-c(jrds  known  a« 
commissures.  The  most  anterior  pair  of  ganglia,  known  as  the 
supra-msophageal  or  cerebral  ganglia,  lie  on  the  dorsal  ii,si)ect  of 
the  pharynx,  a  short  distance  behind  the  anterior  extremity 
(Figs.  24,  29),  From  each  of  them  a  slender  cord,  the  circum- 
msophageal  commissui'e^  passes  dow^n  at  the  side  of  the  pharynx 
to  end  in  the  sub-oesophageal  or  first  ventral  ganglion  on  the 
lower  side,  forming  with  its  fellow  a  complete  ring  or  pharyn- 
geal collar  around  the  alimentary  canal.  From  the  sub-(jesoi)lia- 
geal  ganglion  a  long  double  ventral  nerve-cord  proceeds  back  weirds 
in  the  middle  ventral  line.  The  ventral  cord  consists  of  a  series 
of  double  ganglia,  one  to  each  somite,  connected  by  conunissures 
and  giving  off  lateral  nerves.^ 

Internally  the  cerebral  ganglia  and  the  ventral  cord  (com- 
missures as  well  as  ganglia)  consist  of  both  nerve-cells  and  nerve- 
fibres  as  described  on  p.  U-i. 

Peripheral  Nervous  System.  To  and  from  the  central  sys- 
tem just  described  run  the  nerves  wdiich  constitute  the  peripheral 
system.     These  are  as  follows  : 

1.  A  pair  of  nerves  running  out  on  either  side  of  each  ven- 
tral ffano^lion  and  lost  to  \dew  amons^  the  muscles  of  the  ])ody- 
wall. 

2.  A  single  nerve  proceeding  from  the  ventral  commissures 
on  each  side  immediately  behind  the  dissepiment  to  which  it  is 
mainly  distributed. 

8.   A  pair  of  nerves  from  the  sub-03sophageal  ganglion. 

4.  A  nerve  from  each  half  of  the  pharyngeal  collar  just 
beyond  its  divergence  from  its  fellow.  (Origin  incorrectly 
shown.) 

5.  Two  large  cerebral  nerves,  which  run  forwards  from  the 


*  So  closely  are  tlie  two  halves  of  the  ventral  cord  uniteil  that  its  double 
nature  can  scarcelv  be  made  out  without  sections. 


66 


THE  BIOLOGY  OF  AN  ANIMAL. 


U.TV.C. 

FlQ.  29.— Anterior  portion  of  the  earthworm  laid  open  from  above,  with  the  alimen- 
tary and  circulatory  systems  dissected  away,  c.c,  circum-oesophageal  com- 
missure ;  e.g.,  cerebral  ganglia ;  d*%  dissepiment :  /,  funnel  of  nephridium ;  np 
nephridium ;  o,  ovary ;  od,  oviduct ;  p^i,  pharynx  ;  ps,  prostomium ;  r.s.,  seminal 
receptacle;  s.d.^  sperm-duct;  s.f.,  sperm-funnel;  s.vi.l.^  lateral  seminal  vesicle; 
t,  testis ;  r.g.,  and  v.ii.c.,  ventral  nerve-cord. 


NERVE-IMPULSES.  67 

cerebral   ganglia,  l)reak   up  into   many  brandies,  and  arc*  dis- 
tributed to  the  anterior  part  of  the  body. 

Besides  the  main  gaiij^jlia  of  the  central  system,  tliere  arc  many  smaller 
ganglia  in  various  parts  of  the  body.  Of  these  the  most  important  are  the 
pharyngeal  ganglia—^  to  5  in  niunber— which  lie  on  the  wall  cf  the 
pharynx  on  each  side  just  within  the  i)haryngeal  collar.  They  are  con- 
nected with  the  latter  by  fine  branches,  and  send  minute  nerves  out  upon 
the  walls  of  the  pharynx.  This  series  of  ganglia  is  often  inappropriately 
called  the  sympathetic  system. 

Physiology  of  the  Nervous  System.  Nerve- impulses, 
"What  is  the  origin  and  nature  of  a  nerve-impulse?  lender  nor- 
mal conditions  the  impulse  is  set  up  as  the  result  of  some  dis- 
turbance, technically  called  a  sthnulus^  acting  upon  the  end  of 
the  fibre.  A  touch  or  pressure  upon  the  skin,  for  example,  acts- 
as  a  stinmlus  to  the  nerve-fibres  ending  near  the  point  touchetl — 
that  is,  it  causes  nerve-impulses  to  travel  inwards  along  the  fibres 
towards  the  central  system.  The  nerves  may  be  stimulated  by 
a  great  variety  of  agents : — by  mechanical  disturbance,  as  in  the 
case  just  cited,  by  heat,  electricity,  chemical  action,  and  in 
special  cases  by  waves  of  light  or  of  sound,  and  upon  this  prop- 
erty of  tlie  nerves  depends  the  power  of  the  worm  to  receive  as- 
afferent  impulses  impressions  from  the  outer  world.  But,  besides 
this,  nerve-fibres  may  also  be  stimulated  by  physiological  clianges 
taking  place  within  the  nerve-cells,  which  may  thus  send  out 
efferent  impulses  to  the  various  organs  and  so  control  their  ac- 
tion. 

Regarding  the  precise  nature  of  the  nerve-impulse  we  are  ignorant,  but 
it  is  probably  a  chemical  or  molecular  change  in  the  protoplasm,  travelling- 
rather  rapidly  along  the  fibre,  like  a  wave.*  We  know  that  tJje  nature 
of  the  impulse  is  not  in  any  way  dependent  upon  the  character  of  the  stimu- 
lus. The  stimulus  can  only  throw  the  nerve  into  action  ;  and  this  action 
is  always  the  same  whatever  be  the  stimulus — as  the  action  of  a  clock 
remains  the  same  whether  it  be  driven  by  a  weight  or  by  a  spring. 

Co-ordination.  The  activities  of  tlie  various  organs  are  co- 
ordinated by  a  chain  of  events  which  in  its  sinijilest  f«nMn  is  known 
as  a  rejlex  action^  and  which  lies  at  the  bottom  of  most  of 
the  more  complicated  forms    of  nervous  action.      Its  nature  is 

*  In  the  frog  the  nervous  impulses  travel  at  the  rate  of  about  28  metres  j)er 
second  ;  in  man  it  is  considerably  more  rapid. 


68 


THE  BIOLOGY  OF  AN  ANIMAL. 


illustrated     by     the     diagram  (Fig.    30).       Co-ordination   be- 
tween S  and  M  (two  organs)  is  not  effected  by  a  direct  nervous 

connection,  but  indirectly 
through  a  nerve-centre,  C^ 
which  is  a  nerve- cell  or  group 
of  nerve-cells  situated  in  one 
of  the  ganglia,  with  which  both 
S  and  M  are  separately  con- 
nected by  nerve-libres.  If  S 
be  thrown  into  action,  an  affer- 
ent mipulse  travels  to  C^  ex- 
cites   the     nerve-centre,     and 

Fig.  30.— Diagram  of  simple  reflex  action.  caUSCS   an     efferent    impulse    tO 

S  skin  to  which  stimulus  is  applied ;  a/,  ^^.^^^^  ^^^  ^^  j^  ^j^-^j^  -g  ^j^^^.^, 

the  afferent  nerve-fibre ;  C,  nerve-centre ;  ' 

e/,  efferent  nerve-fibre;   M,  muscle   in  hy  thrOWll    intO    actioil   alsO,  Or 

which  the  efferent  fibre  ends.  .              TXi     i   •                    xx         i.' 

IS  modmed  m  respect  to  actions 
^.Iready  going  on.  Thus  the  actions  of  S  and  M  are  co-ordi- 
nated through  the  agency  of  C\  the  whole  chain  of  events 
•constituting  a  reflex  action. 

For  example,  let  S  be  the  skin  and  If  a  certain  group  of 
muscles.  If  the  skin  be  irritated,  afferent  impulses  travel  in- 
wards to  nerve-centres  in  the  ganglia  (C),  which  thereupon  send 
forth  efferent  impulses  to  the  appropriate  muscles.  Muscular 
contractions  result,  and  the  worm  draws  back  from  the  unwel- 
come irritation. 

This  chain  of  events  involves  three  distinct  actions  on  the 
part  of  the  nervous  system  which  must  be  carefully  distinguished, 
viz.  :  (a)  the  afferent  impulse ;  (h)  action  of  the  centre ;  (c) 
the  efferent  impulse.  It  must  not  be  supposed  that  the  afferent 
Impulse  passes  unchanged  out  of  the  centre  as  the  efferent  impulse, 
i.e.,  is  simply  "reflected,"  like  a  ball  thrown  against  a  wall,  as 
the  word  ' '  reflex ' '  seems  to  imply.  The  afferent  impulse  as  such 
ends  with  the  nerve-centre,  wliich  it  throws  into  activity.  The 
efferent  impulse  is  a  new  action  set  up  by  the  agency  of  the 
centre. 

There  is  reason  to  believe  that  many  if  not  all  nerve-centres 
are  connected  with  a  number  of  different  afferent  and  efferent 
paths,  and  also  with  other  centres,  as  sliown  in  the  diagram 
Fig,    31.      Efferent  impulses  may   therefore    be  sent  out  from 


SENSES  OF  THE  EARTH-  WORM.  69 

tlie  centre  in  various  directions,  and  the  precise  i)atli  cliosen 
depends  on  some  unknown- 
action  taking  place  in  the 
centre.  The  action  of  the 
centre  moreover  may  be 
modified  by  efferent  iin})ulses 
arriving  from  other  centres, 
and  thus  we  can  dimly  per- 
ceive how  reflexes  may  be- 
contr oiled  and  guided,  and 
how  even  the  most  compli- 
cated forms  of  nervous  ac- 
tivity may  be  compounded  Fio.  Sl.-Diagram  representing  three  nerve- 
•^  ^  ^^  ^  centres  and  connections.    Arrows  represent 

out     of     elements     similar     to       the  possible   direction  of    nerve-impulses, 
reflex  actions  ^•^'  °^^  afferent  path ;  e/,  one  efferent  path. 

There  is  reason  to  believe  that  in  the  earthworm  each  ven- 
tral ganglion  presides  over  the  somite  to  which  it  belongs,  and 
is  probably  in  the  main  a  collection  of  reflex  centres  from  wlmse 
action  the  element  of  consciousness  is  absent.  But  there  is  also 
some  reason  to  believe  that  the  cerebral  ganglia  occupy  a  higlier 
position,  since  they  probably  receive  the  nerves  of  sight,  taste, 
and  smell,  besides  those  of  touch,  while  the  ventral  ganglia  re- 
ceive only  those  of  touch.  Exj^eriment  has  shown  furtlier  tluit 
the  cerebral  ganglia  exercise  to  a  certain  limited  extent  a  cou- 
trollinfi^  action  over  those  of  the  ventral  chain  bv  means  of  im- 
pulses  sent  backwards  through  the  commissures,  though  tliis 
action  is  far  less  conspicuous  here  than  in  higher  metameric  ani- 
mals such  as  the  insects.* 

The  Sensitive  System.  (Organs  of  Sense.)  The  sensitive 
system  is  distinguished  from  the  nervous  system  as  a  matter  of 
convenience  of  description,  since  most  of  the  iiigher  animald 
possess  definite  "sense-organs"  which  receive  stimuH  and  thntw 
into  action  the  sensory  nerves  proceeding  from  tlieni.  Ahlioiigh 
the  earthworm  possesses  the  "senses"  of  tuueh,  taste,  sight, 
and  smell,  it  has  no  special  organs  for  these  senses  apart  from 
the  general  integument  covering  the  surface  of  the  Itodv.  and 

*  For  a  fuller  discussion  the  student  is  referred  to  special  works  on  IMivsi- 
ology. 


70  THE  BIOLOGY  OF  AN  ANIMAL. 

hence  can  hardly  be  said  to  possess  any  proper  sensory  system. 
We  do  not  know,  moreover,  whether  the  so-called  "sensations" 
of  the  earthworm  are  really  states  of  consciousness  as  in  ourselves, 
for  we  do  not  even  know  whether  earthworms  possess  any  form 
of  consciousness.  When,  therefore,  we  speak  of  the  earthworm 
as  possessing  the  ' '  sense ' '  of  touch  or  of  sight  we  mean  simply 
that  some  of  the  nerves  terminating  in  the  skin  may  be  stimu- 
lated l)y  mechanical  means  or  by  rays  of  light,  without  necessa- 
rily implying  that  the  worm  actually  feels  or  sees  as  we  feel  and 
see. 

It  has  recently  been  shown  that  the  skin  contains  many  cells  each  of 
wliich  gives  off  a  single  nerve-fibre  that  may  be  traced  directly  into  the 
ventral  nerve-cord.  These  "sensory  cells  "  may  be  regarded  as  "end- 
organs"  through  which  the  stimuli  are  conveyed  to  the  fibres.  It  has  also 
been  shown  that  tliese  cells  are  aggregated  in  minute  groups  thickly  scat- 
tered over  the  surface  of  the  body.  Each  of  these  groups  may  be  regarded 
as  a  simple  form  of  sense-organ. 

The  sense  of  touch  extends  over  the  whole  surface  of  the 
body.  That  of  taste  is  probably  located  in  the  cavity  of  the 
month  and  pharnyx;  the  location  of  the  sense  of  S7nell  is  un- 
known. Darwin's  experiments  have  shown  that  the  earth- 
worm's feeble  sense  of  sight  is  confined  to  the  anterior  end  of 
the  ])ody.  It  is  probable  that  the  nerves  of  sight,  taste,  and 
smell  enter  the  cerebral  ganglia  alone,  while  those  of  touch  run 
to  other  ganglia  as  well. 

Systems  of  (Organs  of)  Support,  Connection,  Protection,  etc. 
The  structure  and  mode  of  life  of  many  animals  are  such  as  to 
require  some  solid  support  to  the  soft  parts  of  the  body.  Such 
supporting  structures  are,  for  instance,  the  bones  of  vertebrata, 
the  hard  outer  shell  of  the  lobster  or  beetle,  and  the  coral 
which  forms  the  skeleton  of  a  polyp.  The  earthworm  has, 
however,  nothing  of  the  sort,  and  it  is  obvious  that  a  hard  sup- 
porting-organ would  be  not  only  useless,  but  even  detrimental. 
The  power  of  creeping  and  burrowing  through  the  earth  depends 
upon  great  flexibility  and  extensibility  of  the  body;  and  with 
this  the  presence  of  a  skeleton  might  be  incomjmtible. 

The  connecting  system  consists  simply  of  various  tissues  by 
which  the  different  organs  are  bound  firmly  together.  These 
can  only  be  seen  upon  microscopical  examination.  The  most 
important  of  them  is  known  as  connective  tissue. 


DEFENCES  OF  THE  EARTHWORM.  71 

As  to  protective  structures,  tlie  eartliworin  is  prol)a])ly  one  of 
the  most  defenceless  of  animals.  Nevertheless  there  are  certain 
structures  which  are  clearly  for  this  purpose.  The  cuticle  wliicli 
covers  the  surface  is  a  thin  but  tough  memljraiie  wliidi  ju-otects 
the  delicate  skin  from  direct  contact  with  hard  oljjects.  It 
passes  into  the  mouth  and  lines  the  alimentary  canal  as  far  down 
as  the  beo^inninc:  of  the  stomach-intestine.  In  the  irizzard. 
where  food  is  ground  up,  the  cuticle  is  prodigicjusly  thick  and 
tough,  and  must  form  a  very  effective  protection  for  tlie  soft 
tissues  beneath  it.  The  main  defence  of  the  animal  lies,  how- 
ever, not  in  any  special  armor,  but  in  those  instincts  which  lead 
it  to  lie  hidden  in  the  earth  during  the  day  and  to  venture  forth 
only  in  the  comparative  safety  of  darkness. 


CHAPTER  Y. 

THE  BIOLOGY  OF  AN  ANIMAL  {Continued). 

The  Earthworm. 
Reproduction.     Embryology. 

Reproduction.  The  life  of  every  organic  species  runs  in 
regularly  recurring  cycles,  for  every  individual  life  has  its  limit. 
In  vouth  the  constructive  processes  preponderate  over  the  de- 
structive and  the  organism  grows.  Tlie  normal  adult  attains  a 
state  of  apparent  physiological  balance  in  which  the  processes  of 
waste  and  repair  are  approximately  equal.  Sooner  or  later, 
however,  this  balance  is  disturbed.  Even  though  the  organism 
escapes  every  injury  or  special  disease  the  constructive  process 
falls  behind  the  destructive,  old  age  ensues,  and  the  individual 
dies  from  sheer  inability  to  live.  Why  the  vital  machine  should 
thus  w^ear  out  is  a  mystery,  but  that  it  has  a  definite  cause  and 
meaning  is  indicated  by  the  familiar  fact  that  the  sjDan  of  natural 
life  varies  with  the  species ;  man  lives  longer  than  the  dog,  the 
elephant  longer  than  man. 

It  is  a  w^onderful  fact  that  living  things  have  the  power  to 
detach  from  themselves  portions  or  fragments  of  their  own 
bodies  endowed  with  fresh  powers  of  growth  and  develoj^ment 
and  capable  of  running  througli  tlie  same  cycle  as  the  parent. 
There  is  therefore  an  unbroken  material  (protoplasmic)  continuity 
from  one  generation  to  another,  that  forms  tlie  physical  basis  of 
inheritance,  and  upon  which  the  integrity  of  the  s]3ecies  depends. 
As  far  as  known,  li\dng  things  never  arise  save  through  this 
process;  in  other  words  every  mass  of  existing  protoplasm  is 
the  last  link  in  an  unbroken  chain  that  extends  backward  in  the 
past  to  the  first  origin  of  life. 

The  detached  portions  of  the  parent  that  are  to  give  rise  to 
offspring  are  sometimes  masses  of  cells,  as  in  the  separation  of 
branches  or  buds  among  plants,  but  more  commonly  they  are  single 

72 


REPRODUCTION.  73 

cells,  known  as  germ-cells,  like  the  eggs  of  animals  and  tlie 
spores  of  ferns  and  mosses.  Only  the  germ-cells  (which  may 
conveniently  be  distinguished  from  those  forming  the  rest  of  the 
body,  or  the  somatic  cells),  escape  death,  and  that  only  under 
certain  conditions. 

All  forms  of  reproduction  fall  under  one  or  the  other  of  two 
heads,  viz.,  Agamogenesis  {asexual  reproduction)  or  Gamogenesis 
{sexual  reproduction).  In  the  former  case  the  detached  portion 
(which  may  be  either  a  single  cell  or  a  group  of  cells)  has  the 
power  to  develop  into  a  new  individual  without  the  influence  of 
other  living  matter.  In  the  latter,  the  detached  portion,  in  tliis 
case  always  a  single  cell  (ovum,  ousphere,  etc.),  is  acted  u})(>n 
by  a  second  portion  of  living  matter,  likewise  a  single  cell,  wliich 
in  most  cases  has  been  detached  from  the  body  of  another  in- 
dividual. The  germ  is  called  the  y<2m«Z^  germ- cell ;  the  cell  act- 
ing upon  it  the  "male  gerin-cell  /  and  in  the  sexual  process  the 
two  fuse  together  {fertilization,  imjjr  eg  nation)  to  form  a  single 
new  cell  endowed  with  the  power  of  developing  into  a  new  in- 
dividual. In  some  organisms  (e.g.,  the  yeast-plant  and  bacteria) 
only  agamogenesis  has  been  observed ;  in  others  (e.  g. ,  vertebrates) 
only  gamogenesis ;  in  others  still  both  processes  take  place  as  in 
many  higher  plants. 

The  earthworm  is  not  known  to  multiply  by  any  natural 
process  of  agamogenesis.  It  possesses  in  a  high  degree,  however, 
the  closely  related  power  of  regeneration  /  for  if  a  worm  be  cut 
transversely  into  two  pieces,  the  anterior  piece  will  usually  make 
good  or  rege'iierate  the  missing  portion,  while  the  j^osterior  })i(.'ce 
may  regenerate  the  anterior  region.  Thus  the  worm  can  to  a 
certain  limited  extent  be  artificially  propagated,  like  a  plant,  by 
cuttings,  a  process  closely  related  to  true  agamc jgenesis. -^  Its 
usual  and  normal  mode  of  reproduction  is  by  gamogenesis,  that 
is,  by  the  formation  of  male  germ-cells  {sj)er)?iatozoa)  and  female 
germ-cells  (ova).  In  higher  animals  the  two  kinds  uf  germ- 
cells  are  produced  by  different  individuals  of  opi)osite  sex.  The 
earth w^orm  on  the  contrary  is  hermaphrodite  or  lisexual;  e\ery 

*  Many  worms  nearly  related  to  Lumhricns—e.g:.,  the  genus  Duo,  and  other 
Naids — spontaneously  divide  themselves  into  two  parts  each  of  which  becomes 
a  perfect  animal.  This  process  is  true  agamogenesis,  though  obviously  closely- 
related  to  regeneration. 


74 


THE  BIOLOGY  OF  AN  ANIMAL. 


individual  is  hoth  male  and  female^  producing  both  eggs  and 
sjDermatozoa.  The  ova  arise  in  special  organs,  the  ovaries^  the 
spermatozoa  in  spermaries  or  testes. 

The  ripe  ovum  (Fig.  33,  B)  is  a  relatively  large  spherical 
cell,  agreeing  closely  with  the  e^g  of  the  star-lish  (Fig.  12),  but 
having  a  thinner  and  more  delicate  membrane.  It  is  still  cus- 
tomary to  apply  to  ova  the  old  terminology,  calling  the  cell- 
substance  ly'dellus^  the  membrane  vitelline  membrane,  the  nucleus 
germinal  vesicle,  and  the  nucleolus  germinal  spot. 

The  ripe  spermatozoon  (Fig.  33,  6^)  is  an  extremely  minute 
elongated  cell  or  filament  thickening  towards  one  end  to  form 
the  head  (71),  which  contains  the  nucleus  of  the  cell  enveloped  by  a 
thin  layer  of  protoplasm.  This  is  followed  by  a  short  ' '  middle 
piece  ' '  (m)  to  which  is  attached  a  long  vibratory  fiagellum  or  tail 
(t).  The  tail  is  virtually  a  long  cilium  (p.  31),  which  by  vigorous 
lashing  di'ives  the  whole  cell  along  head-foremost,  very  much  as 
a  tadpole  is  driven  by  its  tail. 

Since  the  ovaries  and  spermaries  give  rise  to  the  germ-cells, 

they  are  called  the  essential  organs  of 
reproduction.  Besides  these,  ZtimhricuSj 
like  most  animals,  has  accessory  organs  of 
reproduction  which  act  as  reservoirs  or 
carriers  of  the  germs,  assist  in  securing 
cross-fertilization,  and  minister  to  the 
wants  of  the  young  worms. 

Essential  Reproductive  Organs.  The 
ovaries  are  two  in  number  and  lie  one  on 
eitlier  side  in  the  13th  somite  attached  to 
the  hinder  face  of  the  anterior  dissepiment 
{ov,  Fig.  29).  They  are  about  2"^"^  in 
length,  distinctly  pear-shaped,  and  at- 
tached by  the  broader  end  (Fig.  32).  The 
narrow^  extremity  contains  a  single  row  of 
ova  and  is  called  the  eqq-strinq  ies).      In 

Fig.  32.— The  ovary,   much      ,  .       ,  .  ^^        ^   \     J 

enlarged,  /j,  the  basal  part;  this  the  ova  are  ripe  or  nearly  so;   behind 

a  body  of  the  ovary  con-    ^|^        g|^^^g   ^^    -^^^^   ^l^^g^  ^^^^   ^^^ 
taming  immature  ova;  es,    ^       "^ 

egg-string;  oi\  ripe  ovum  immature,  till  these  are  lost  in  a  mass  of 

rea  >  to  a   off.  nearly   undifferentiated   cells    {primitive 

ova),  constituting  the  great  bulk  of  the  ovary.     Each  of  these, 


ov 


ea 


'V V;  ■' y*'  •■'*:••  ^'>i1 


'^0:iy0Mm 


REPRODUCTIVE   ORGANS.  75 

however,  is  surrounded  witli  still  smaller  cells  constitutiii*'-  its 
nutrient  envelope  or  follicle.  As  the  ova  mature  the  follicles 
still  persist,  and  they  may  be  detected  even  in  the  ci^j^sti-iiiir. 
When  fully  ripe  the  ovum  bursts  the  follicle  and  is  shed  from 
the  end  of  the  egg-string  into  the  body-cavity.  It  is  ultimately 
taken  into  the  oviduct  and  carried  to  the  exterior. 

The  development  of  the  ovary  shows  it  to  be  morphologically 
a  thickening  of  the  peritoneal  epithelium.  The  eggs  therefore 
are  originally  epithelial  cells. 

The  sjjennaries  or  testes  {t,t,  Fig.  29)  are  four  in  mnnl)eraiid 
in  outward  appearance  are  somewhat  similar  to  the  ovaries. 
They  are  small  ilattened  bodies  with  somewhat  irregular  or  lohed 
borders,  lying  one  on  either  side  the  nerve-cord  in  a  position 
corresponding  with  that  of  the  ovaries,  but  in  somites  10  and  11. 
Like  the  ovary  the  testis  is  a  solid  mass  of  cells,  which  are  shed 
into  the  body-cavity  and  are  finally  carried  to  the  exterior. 
The  sperm-cells  leave  the  testis,  however,  at  a  very  early  period 
and  undergo  the  later  stages  of  maturation  within  the  cavities  of 
the  seminal  vesicles  described  below. 

Accessory  Reproductive  Organs.  The  most  important  of  the 
accessory  organs  are  the  genital  ducts,  by  which  the  germ-cells 
are  passed  out  to  the  exterior.  Both  the  female  ducts  {oviducts) 
and  the  male  {sperm-ducts)  are  tubular  organs  opening  at  one 
end  to  the  outside,  through  tlie  body-wall,  and  at  the  other  end 
into  the  coelom  by  means  of  a  ciliated  funnel  somewhat  similar 
to  a  nephridial  funnel,  but  nnicli  larger.  By  means  of  these 
ciliated  funnels  the  germ-cells  after  their  discharge  t'runi  the 
ovary  or  testis  are  taken  up  and  passed  to  the  exterior. 

The  oviducts  {od,  Fig.  29,  Fig.  23)  are  two  short  trumpet- 
shaped  tubes  lying  innnediately  posterior  to  the  ovaries  and  p;uss- 
ing  through  the  dissepiment  between  the  13th  and  14th  somites. 
Tlie  inner  end  opens  freely  into  the  cavity  of  the  13th  somite, 
by  means  of  a  wide  and  much-folded  ciliated  funnel,  fi-om  the 
centre  of  which  a  slender  tube  passes  backward  througli  the 
dissepiment,  turns  rather  sharply  towards  the  outer  side  and, 
passing  through  the  body-wall,  opens  to  the  outside  on  the  14th 
somite  (see  p.  43).  Innnediately  behind  the  dissepiment  the 
oviduct  gives  off  at  its  dorsal  and  outer  side  a  small  ])oueh, 
richly  supplied  with  blood-vessels.      In  this,   the  receptacul um 


76  THE  BIOLOGY  OF  AN  ANIMAL. 

ovorum^  the  ova  taken  up  by  the  funnel  are  temporarily  stored 
before  passing  out  to  the  exterior. 

It  is  probable  tliat  the  eggs  never  float  freely  in  the  coelom, 
but  drop  out  of  the  ovary  at  maturity  directly  into  the  mouth  of 
the  funnel,  Tliey  pass  thence  into  the  receptaculum^  where  they 
may  remain  for  a  considerable  period. 

The  sperm-ducts  {vasa  deferentia)  {sd,  Fig.  29)  are  very 
long  slender  tubes,  open  like  the  oviducts  at  both  ends.  The 
outer  opening  is  a  conspicuous  slit  surrounded  by  fleshy  lips 
(Fig.  21),  on  the  ventral  side  of  the  loth  somite.  From  this 
point  the  duct  runs  straight  forwards  to  the  12tli  somite,  where 
it  branches  like  a  Y,  the  two  branches  passing  forwards  to  ter- 
minate, one  in  the  11th  somite,  the  other  in  the  10th.  I^s^ear  its 
end  each  branch  is  twisted  into  a  peculiar  knot  and  Anally  ter- 
minates in  an  innnense  ciliated  funnel  (the  so-called  "ciliated 
rosette"),  the  borders  of  wliicli  are  folded  in  so  complicated  a 
manner  tliat  they  form  a  labyrinthine  body,  the  true  nature  of 
wliieh  can  only  be  made  out  in  microscopic  sections. 

The  two  pairs  of  sperm-funnels  (Fig.  29)  lie  in  the  10th 
and  11th  somites,  innnediately  posterior  to  the  respective  testes, 
i.e.,  they  have  essentially  the  same  relation  to  the  testes  as  that 
of  the  oviduct-funnels  to  the  ovaries. 

The  testes  and  sperm-funnels  can  be  readily  made  out  only  in  young 
specimens.  In  mature  worms  they  are  completely  enveloped  by  the  semi- 
nal vesicles  described  below. 

Seminal  vesicles.  Tliese,  the  most  conspicuous  part  of  the 
reproductive  apparatus,  are  A^oluminous  pouches  in  which  the 
spenn-cells  undergo  their  later  development,  after  leaving  the 
testis.  They  are  large  white  bodies  lying  in  somites  9  to  12  and 
usually  overlapping  the  oesophagus  in  that  region.  In  all  cases 
there  are  three  pairs  of  lateral  seminal  vesicles,  viz.,  an  anterior 
pair  in  somite  9,  a  middle  pair  in  somite  11,  and  a  posterior  pair 
in  somite  12.  In  unmature  specimens  tliese  six  are  entirely 
separate,  and  allow  tlie  testes  to  be  easily  seen.  In  mature 
worms  (as  shown  in  Fig.  29)  the  posterior  pair  of  lateral 
vesicles  grow  together  in  the  middle  line,  thus  forming  a  2)os- 
terior  median  vesicle  lying  below  the  alimentary  canal  in  the 
11th  somite.  In  like  manner  an  anterior'  median  vesicle  is 
formed  in  the  10th  somite  by  the  union  of  the  two  anterior  pairs 


EGG-LA  YING.  77 

of  lateral  vesicles.  The  two  median  vesicles  thus  formed  eiivehji) 
the  testes  and  sperm-funnels  uf  their  respective  somites  and  hide 
them  from  view. 

The  sperm-cells  leave  the  testis  at  a  very  early  period  and  float  freely 
in  the  cavities  of  the  seminal  vesicles,  where  many  stages  of  their  develop- 
ment may  easily  be  observed.  They  are  developed  in  balls  known  as 
spermatospheves,  each  of  which  consists  of  a  central  solid  mass  of  proto- 
plasm surrounded  by  a  single  layer  of  sperm-cells.  When  mature  the 
spermatozoa  separate  from  the  central  mass  and  are  drawn  into  the  fun- 
nels of  the  sperm-ducts.  The  manner  in  which  this  action  is  controlled  is 
not  understood. 

The  seminal  rece;ptacles  are  accessory  organs  of  reproduction 
in  the  shape  of  small  rounded  sacs  or  potiches,  open  to  the  out- 
side only,  at  about  the  level  of  the  upper  row  of  set*.  They 
lie  between  the  9th  and  10th,  and  lOtli  and  lltli  somites  (*./•, 
Figs.  24  and  29),  where  their  openings  may  be  sought  for  (Fig. 
21).      Their  function  is  explained  under  the  head  of  co])ulati()n. 

Accessory  glands.  Besides  all  the  structures  so  far  described 
there  are  many  glands  which  play  a  part  in  tlie  reproductive 
functions.  The  setigerous  glands  from  about  the  7th  to  about 
the  19th  somite  (sometimes  fewer,  sometimes  none  at  all)  are 
often  greatly  enlarged,  and  form  the  glandular  j^rominences  men- 
tioned at  p.  46.  They  seem  to  be  used  as  organs  of  adhesion 
during  copulation.  The  clitellum  is  tilled  with  gland-cells  which 
probably  serve  in  j)art  to  secrete  a  nourishing  tluid  for  the  young 
worms,  and  in  part  to  provide  a  tough  protecting  membrane  to 
cover  them. 

Copulation.  Egg-laying^.  Inasmuch  as  each  individual  earth- 
worm produces  both  ova  and  spermatozoa,  it  might  be  supposed 
that  copulation,  or  the  sexual  union  of  two  different  individuals, 
would  not  be  necessary.  This,  however,  is  not  the  case.  The 
ova  of  one  individual  are  invariably  fertilized  by  the  s])ermatozoa 
of  another  individual  after  a  process  of  copulation  and  exchange 
of  spermatozoa,  as  follows :  During  the  night-time,  and  usually 
in  the  spring,  the  worms  leave  their  burrows  and  ])air.  placing 
themselves  so  that  their  heads  point  in  opposite  directions  and 
holding  firmly  together  by  the  enlarged  setigerous  glands  and  the 
thickened  lower  lateral  margins  of  the  clitellum.  During  this 
act  the  seminal  receptacles  of  each  w<^rm  are  filled  witli  s]>erma- 
tozoa  from  the  sperm-ducts  of  the  other,  after  which  the  wnrms 


\7I% 


78  THE  BIOLOGY  OF  AN  ANIMAL. 

separate.  [The  spermatozoa  thus  received  are  simply  stored  up 
and  do  not  perform  their  function  until  the  time  of  egg-laying.] 
When  the  worm  is  ready  to  lay  its  eggs  the  glands  of  the 
clitelJmn  become  very  active,  pouring  out  a  thick  glairy  fluid 
which  soon  hardens  into  a  tough  membrane  and  forms  a  girdle 
around  the  body.  Besides  this  a  large  quantity  of  a  thick  jelly - 
like  nutrient  fluid  is  poured  out  and  retained  in  the  s]3ace  be- 
tween the  gu-dle  and  the  body  of  the  worm.  The  girdle  is 
thereupon  gradually  worked  forward  toward  the  head  of  the 
worm  by  contractions  of  the  body.  As  it  passes  the  1-ith  somite 
a  number  of  ova  are  received  from  the  oviducts,  and  between 
the  9th  and  11th  somites  a  quantity  of  spermatozoa  are  added 
from  the  seminal  receptacles  where  they  have  been  stored  since 
the  time  of  copulation,  when  they  were  obtained  from  another 
worm.     The  girdle  is  next  stripped  forwards  over  the  anterior 

end  and  is  finally  thrown 
completely  off.  As  it 
passes  off  its  oj^en  ends 
immediately  contract 
tightly  together,  and  the 
girdle  becomes  a  closed 
capsule  (Fig.  33)  contain- 
ing both  ova  and  sj^erma- 
tozoa  floatinoj  in  a  nutri- 

Fio.  33.—^,  egg-capsule  enlarged  5  diameters      .  n    •  j  'n  m 

(a  few  eggs,  01%  enlarged  to  the  same  scale  are    tive    liuiQ    Or    milk.        Ihe 

shown  near  by  on  the  right)  ;B,  an  ovum  very    niembraiie   SOOU   aSSUmCS  a 

much  enlarged ;  C,  a  spematozoon,  enormously 

magnified  ;  71,  head  ;  m,  middle  piece ;  f,  tail.        light   yellowisll    Or    Lrown 

color,  becomes  hard  and  tough,  and  serves  to  protect  the  de- 
veloping embryos.  The  capsules  may  be  found  in  Iviay  or  June 
in  earth  under  logs  or  stones,  or  especially  in  heaps  of  manure. 
Within  the  capsules  the  fertilization  and  development  of  the  ova 
take  place. 

Fertilization  and  Embryological  Development.  The  s]3erma- 
tozoa  swim  actively  about  in  the  nutrient  fluid  of  the  capsule^ 
approach  an  ovum,  and  attach  themselves  to  its  surface  by  their 
heads.  Several  of  the  spermatozoa  then  enter  the  vitellus  (cf . 
p.  80),  but  it  has  been  proved  that  only  one  of  these  is  con- 
cerned in  fertilization,  the  others  dying  and  becoming  absorbed 
by  the  ovum. 


B 


FERTILIZATION  OF  THE  EGO. 


79 


It  is  probable  that  the  tail  plays  no  part  in  the  actual  fertili- 
zation, but  is  merely  a  locomotor  apparatus  fur  tlie  head  (nucleus) 
and  middle-piece. 

Within  the  ovum  the  head  of  the  spermatozoun  })ersi8ts  as 
the  s^erin-nucleus  (or  male  pro-nucleus)^  while  the  protoplaMn  in 
its  neighborhood  assumes  a  peculiar  and  characteristic  ra<liate 
arrangement  like  a  star,  probably  through  the  influence  of  the 
middle-piece. 

After  the  entrance  of  the  spermatozoon  the  ^g'g  segments  ulf 


Fig.  34.— Fertilization  of  the  ovum.  A.,  entrance  of  the  spermatozoon  (in  the  sea- 
urchin,  after  Fol).  B,  the  sea-urchin  egg  after  entrance  of  the  sperniatozoim ; 
within  and  to  the  left  is  the  egg-nucleus;  above  is  the  sperm-nucleus,  with  a  cen- 
trosomenear  it  (modified  from  Hertwig).  r,  diagram  of  the  ovum  after  extrusion 
of  the  polar  cells  (p.c),  and  union  of  the  two  pro-nuclei  to  form  the  segmenta- 
tion-nucleus. The  smaller  and  darker  portion  of  the  latter  is  derived  from  the 
sperm-nucleus.  Two  asters  or  archoplasm-spheres  are  shown  near  the  nuiU-us. 
These  arise  by  the  division  of  a  single  aster  derived  from  the  middle-piece  of  the 
spermatozoon.  D,  two-celled  stage  of  the  earthworm,  after  the  first  fission  of 
the  ovum.    (After  Vejdovsky.) 

at  one  side  two  small  cells,  one  after  the  other,  known  as  the 
^olar  cells  oy  jyolar  hodles.  These  take  no  part  in  tlie  formation 
of  the  embryo,  and  their  formation  prol)ab]y  serves,  in  some  way 
not  yet  wholly  clear,  to  prej)are  tlie  egg  for  the  lju>t  act  of 
fertilization.  After  the  formation  of  the  polar  cells  the  egg- 
nucleus  (now  often  called  t\\.Q  female pi'o-nucle us)  and  tlie  sperm- 
nucleus    approach   one   another    and    iinally   become    intimately 


80  THE  BIOLOOT  OF  AN  ANIMAL. 

associated  to  form  tlie  segmentation-  or  cleavage-nucleus  ;  by  this 

act  fertilization  is  completed. 

The  process  of  fertilization  appears  to  be  essentially  the  same  among 
all  higher  animals,  and  in  a  broader  sense  to  be  identical  with  the  sexual 
process  among  all  higher  and  many  lower  plants  (compare  the  fern,  p.  139), 
but  its  precise  nature  is  still  in  dispute.  It  is  certain  that  one  essential 
part  of  it  is  the  union  of  two  nuclei  derived  from  the  two  respective  parents. 
This  has  led  to  the  view,  now  held  by  many  investigators,  that  inheritance 
has  its  seat  in  the  nucleus,  and  that  chromatin  (p.  23),  is  its  physical 
basis.  Later  researches  have  shown  that  another  element  known  as  the 
archoplasm-  or  attraction-sphere  is  concerned  in  fertilization,  and  this  is 
apparently  always  derived  from  the  middle-piece.  It  is  not  yet  certain 
whether  the  archoplasm  is  to  be  regarded  as  a  nuclear  or  a  cytoplasmic 
structure,  and  it  is  equally  doubtful  whether  it  plays  an  essential  or  merely 
a  subsidiary  role  in  fertilization  and  inheritance  (cf.  p.  84). 

Cleavage  of  the  Fertilized  Ovum.  Soon  after  fertilization  the 
ovum  begins  the  remarkable  process  of  segmentation  which 
has  already  been  briefly  sketched  on  j).  25.  The  segmen- 
tation-nucleus divides  into  two  parts,  and  this  is  followed  by 
a  division  of  the  vitellus,  each  half  of  the  original  nucleus  becom- 
ing the  nucleus  of  one  of  the  halves  of  the  vitellus ;  that  is,  the 
original  cell  divides  into  two  smaller  but  similar  cells  (see  Fig. 
3-1:).  These  divide  in  turn  into  four,  and  these  into  eight,  and 
so  on,  but  yet  remain  closely  connected  in  one  mass.  In  the 
case  of  the  earthworm,  the  cells  do  not  multiply  in  regular 
geometrical  progression,  but  show  many  irregularities ;  and  more- 
over they  become  unequal  in  size  at  an  early  period. 

The  blastula  (pp.  25,  85,)  shows  scarcely  any  differentiation 
of  parts,  though  the  cells  of  one  hemisphere  are  somewhat  smaller 
than  the  others.  From  this  time  forwards  the  whole  course  of 
development  is  a  process  of  differentiation,  both  of  the  cells  and  of 
the  organs  into  wliicli  they  soon  arrange  themselves.  One  of 
the  first  steps  in  this  process  is  a  flattening  of  the  embryo  at  the 
lower  pole — i.e.,  the  half  consisting  of  larger  cells  (Fig.  35,  D), 
The  large  cells  are  then  folded  into  the  segmentation-cavity  so 
as  to  form  a  pouch  opening  to  the  exterior ;  at  the  same  time 
the  embryo  becomes  somewhat  elongated  (Fig.  35,  ^,  F). 

This  process  is  known  as  gastrulation^  and  at  its  completion 
the  embryo  is  called  the  gastrula.  The  infolded  pouch  (called 
the  archenteron)  is  the  future  alimentary  canal ;  its  opening  (now 
known  as  the  hlastopoi^e)  will  become  the  mouth ;  and  tlie  layei 


THE  GERM-LAYERS. 


81 


of  small  cells  over  the  outside  will  form  the  skin  or  (\\\X{'y  laver 
of  the  body-wall. 

The  embryo  very  soon  begins  to  swallow,  through  the  bla-sto- 
pore,  the  milkhke  tiuid  in  which  it  tloats,  and  to  di<»"est  it  with- 
in the  cavity  of  the  archenteron. 

It  is  obvious  that  the  embryo  already  shows  a  distinct  (hlfcr- 


B 


Fig.  35.— Diagrams  of  the  early  stages  of  development  in  the  earthworm.  .1,  accu- 
rate drawing  of  the  blastula,  surrounded  by  the  vitelline  membrane  i after  Vej- 
dovsky) ;  B,  blastula  in  optical  section  showing  the  large  segmentation-t-avity 
{s.c.\  and  the  parent-cell  of  the  mesoblast  (m.);  C,  later  blastula,  showing  forma- 
tion of  mesoblast-cells ;  D,  flattening  of  the  blastula  preparatory  to  invagination  ; 

E,  the  gastrula  in  side  view;  as  the  infolding  takes  place  the  two  niesoblast- 
bands  are  left  at  the  sides  of  the  body,  in  the  position  shown  by  the  dotted  lines; 

F,  section  of  E  along  the  line  s-s,  showing  the  mesoblast-bands  and  pole-cells. 

entiation  of  parts  which  perform  unlike  functions.  In  fact  we 
may  regard  the  gastrula  as  composed  of  two  tissues  still  nearly 
similar  in  structure  though  unlike  in  function.  One  of  these 
consists  of  the  layer  of  cells  which  forms  the  outer  covering; 
this  tissue  is  known  as  the  ectohlast  {ec\  Fig.  35).  The  second 
tissue  is  the  layer  of  cells  forming  the  wall  of  the  archenteron ; 
it  is  called  the  entoblast  {en).  The  ectoblast  and  eiitol>last  to- 
gether are  known  as  \h(i  primary  germ-layers. 

Meanwhile  changes  are  taking  place  which  result  in  the  for- 
mation of  a  third  germ-layer  lying  in  the  segmentation-cavity 
between  the  ectoblast  and  entoblast  and  therefore  called  the 
mesoblast  {m,  Figs.  35,  30).  In  some  animals  the  mesobltu^t 
does  not  arise   until  after  the   completion  of  gastrulation.      In 


S2 


THE  BIOLOGY  OF  AN  ANIMAL. 


Lumhricus,  however,  it  goes  on  during  gastrulation  and  begins 
€ven  before  gastrulation.  Even  in  the  blastula  stage  two  large 
cells  may  be  distinguished  which  afterwards  give  rise  ^  to  the 
niesoblast  and  are  hence  called  the  primary  mesoblastic  cells. 
They  soon  bud  forth  smaller  cells  hito  the  segmentation- cavity, 
and  as  tlie  blastula  flattens  they  themselves  sink  below  the  sur- 
face.  At  this  period,  therefore,  the  niesoblast  forms  two  bands 
of  cells  {mesoUast-hands)  each  terminating  behind  in  the  large 
mother-cell  or  pole-cell.  Throughout  the  later  stages  the  pole- 
cells  continue  to  bud  forth  smaller  cells  which  are  added  to  the 
hinder  ends  of  the  mesoblast-bands  (Figs.  35,  36). 


Ftg.  36.— Diagrams  of  later  embryonic  stages.  A^  late  stage  in  longitudinal  section, 
showing  the  appearance  of  the  cavities  of  the  somites  ;  JB,  the  same  in  cross-sec- 
tion ;  t;,  diagram  of  a  young  worm  in  longitudinal  section  after  the  formation  of 
the  stomodaeum,  proctodaeum,  and  anus ;  C,  the  same  in  cross-section,  showing 
the  beginning  of  the  nervous  system;  D,  cross-section  of  later  stage  with  the 
nervous  system  completely  established,  a/,  alimentary  canal ;  07%  archenteron ; 
an,  anus;  c<£^  coelom;  cc.,  ectoblast;  en,  entoblast;  ?ni,  primary  mesoblastic  cells; 
m",  mesoblast;  mh.,  mouth  ;  7J,  nervous  system;  s,  cavity  of  somite;  s.w,  somatic 
layer  of  the  mesoblast,  which  with  the  ectoblast  forms  the  somatopleure ;  syA.m, 
splanchnic  layer  of  the  mesoblast,  which  with  the  entoblast  forms  the  splanch- 
nopleure. 

After  each  division  the  pole-cells  increase  in  size,  so  that  up 
to  a  late  stage  in  development  they  may  be  distinguished  from 


CELL-DIVISION.     EAR  70KINESIS. 


83 


the  cells  to  which  thej  give  rise.  The  twu  masses  of  iiiesohlastic 
cells  gradually  increase  in  size  and  finally  fill  the  segmentation- 
cavity. 

The  internal  phenomena  of  cell-division  are  of  great  complexity  and 
can  here  be  given  only  in  outline.  The  ordinary  type  of  cell-division,  as 
shown  in  the  segmentation  of  the  ovum  and  in  the  multiplication  of  most 
tissue-cells,  involves  a  complicated  series  of  changes  in  the  nucleus  known 
as  karyokinesis  or  mitosis.  These  changes,  which  appear  to  be  of  essen- 
tially the  same  character  in  nearly  all  kinds  of  cells,  and  both  in  jjlants  and 
in  animals,  are  illustrated  by  the  following  diagrams  : 


C  D 

Fig.  37.— Diagrams  of  indirect  cell-division  or  karyokinesis. 

A.  Cell  just  prior  to  division,  showing  nucleus  (/j)  with  its  chromatic  reticulum  and 
the  attraction-sphere  and  centrosome  (c). 

B.  First  phase;  the  attraction-sphere  has  divided  into  two,  which  have  moved 
180°  apart;  the  reticulum  has  been  resolved  into  five  chromosomes  (black),  each 
of  which  has  split  lengthwise. 

C.  Second  phase;  fully  developed  karyokinetic  figure  (amphiastcr),  with  spindle 
and  asters;  the  chromosome-halves  are  moving  apart. 

D.  Final  phase;  the  cell-body  is  dividing,  the  spindle  disappearing,  tlie  dauglitor- 
nuclei  about  to  be  formed. 

In  its  resting  state  the  nucleus  contains   a  network  or  reticulum  of 
chromatin  (Fig.  37,  A),     As  the  cell  prepares  for  division  a  small  body  (c) 


84  THE  BIOLOGY  OF  AN  ANIMAL. 

makes  its  appearance  near  the  nucleus,  known  as  i\iQ  attraction- sj:)here  or 
archoplasm-mass,  and  in  its  interior  there  is  often  a  smaller  body,  the 
centrosome.  The  first  step  in  cell-division  is  the  fission  of  the  archoplasm- 
mass  into  two,  each  containing  a  centrosome  (derived  by  fission  of  the 
original  centrosome);  after  this  the  two  masses  move  apart  to  opposite 
poles  of  the  nucleus  (Fig.  37,  B).  The  reticulum  now  becomes,  in  most 
cases,  resolved  into  a  thread  coiled  into  a  skein  (not  shown  in  the  figure), 
which  finally  breaks  up  into  a  number  of  bodies  known  as  cJiromosomes. 
Their  form  (granular,  rodlike,  loop-shaped)  and  number  (two,  eight,  twelve, 
sixteen,  etc.,  or  often  much  higher  numbers)  appear  to  be  constant  for 
each  species  of  plant  and  animal.  The  second  principal  step  is  the  longi- 
tudinal splitting  of  each  chromosome  into  halves  (Fig.  37,  B)  and  the 
disappearance  of  the  nuclear  membrane. 

In  the  third  place  starlike  rays  {aster)  appear  in  the  protoplasm  around 
the  archoplasm-masses,  a  spindle-shaped  structure  appears  between  them 
(Fig.  37,  C),  and  the  double  chromosomes  arrange  themselves  around  the 
equator  of  the  spindle.  The  structure  thus  formed  is  known  as  the  amjjJii- 
aster  or  karyokinetic  figure. 

Fourthly,  the  two  halves  of  each  chromosome  move  apart  towards  the 
respective  poles  of  the  spindle  and  the  entire  cell-body  then  divides  in  a 
plane  passing  through  the  equator  of  the  spindle.  Each  group  of  daughter- 
chromosomes  now  gives  rise  to  a  reticulum,  which  becomes  surrounded  with 
a  membrane  and  forms  the  nucleus  of  the  daughter-cell.  The  spindle  dis- 
appears, and  in  some  cases  the  archoplasm-mass,  with  its  star-rays  (aster), 
seems  to  disappear  also.  In  other  cases,  however,  the  archoplasm-mass  and 
centrosome  persist  and  may  be  found  in  the  resting  cell  (e.g.,  in  leucocytes 
and  connective-tissue  cells),  lying  near  the  nucleus  in  the  cytoplasm. 

It  appears  from  the  foregoing  description  that  each  daughter-cell  re- 
ceives exactly  half  the  substance  of  the  mother-nucleus  (chromatin),  mother- 
archoplasm,  and  mother-centrosome.  In  many  cases  the  cytoplasm  also 
divides  equally,  in  other  cases  unequally. 

It  has  been  proved  in  a  considerable  number  of  cases  that  in  the  fer- 
tilization of  the  ovum  each  germ-cell  contributes  the  same  number  of  chro- 
mosomes, and  the  wonderful  fact  has  been  established  with  high  probability 
that  the  paternal  and  maternal  chromatic  substances  are  equally  distributed 
to  the  two  cells  found  at  the  first  segmentation  of  the  ovum.  It  is  further 
probable  that  this  equal  distribution  continues  in  all  the  later  divisions ; 
and  if  this  is  true,  every  cell  in  the  whole  adult  body  contains  material 
directly  derived  from  both  parents,  and  hence  may  inherit  from  both. 

Gastrulation.  Germ-layers.  Diiferentiation.  Origin  of  the 
Body.  Almost  from  the  first  the  cells  arrange  themselves  so  as 
to  surround  a  central  cavity  known  as  the  segmentation-cavity. 
This  cavity  increases  in  size  in  later  stages,  so  that  the  embryc* 
finally  appears  as  a  hollow  sphere  surrounded  by  a  wall  consist- 


DEVELOPMENT  OF  THE  ORGANS.  85 

ing  of  a  single  layer  of  cells.     This  stage  is  known  as  the  hhutula 
(or  hlastosphere)  (J.,  B^  Fig.  35). 

The  formation  of  the  germ-layers  is  one  of  the  most  im- 
portant and  signillcant  processes  in  the  whole  course  of  devehjp- 
ment.  Germ-layers  like  those  of  Lmnhrlrus^  and  callt-d  hy 
the  same  names,  are  found  in  the  embryos  of  all  higher  ani- 
mals; and  it  will  hereafter  appear  that  this  fact  has  a  })n (found 
meaning. 

Development  of  the  Organs.  (Organogeny.)  The  embry(j  gradu- 
ally increases  in  size  and  at  the  same  time  elongates.  As  it 
lengthens,  the  blastopore  (in  this  case  the  mouth)  remains  at  one 
end,  wliicli  is  therefore  to  be  regarded  as  anterior,  and  the 
elongation  is  backwards.  The  cells  of  all  three  germ-layers 
continually  increase  in  number  by  division,  new  matter  and 
energy  being  supplied  from  the  food,  which  is  swalhjwed  by  the 
embryo  in  such  quantities  as  to  swell  up  the  body  like  a  bladdrr. 
The  archenteron  enlarges  until  it  comes  into  contact  with  the 
ectoblast  and  the  segmentation-cavity  is  obliterated. 

The  two  primary  mesoblastic  cells  are  carried  backwards, 
and  always  remain  at  the  extreme  posterior  end  (???,  Fig.  .ST)). 
The  mesoblast  is  in  the  form  of  tw^o  bands  Ivino:  on  either  side 
of  the  archenteron,  and  extending  forwards  from  the  primary 
mesoblastic  cells. 

This  is  clearly  seen  in  a  cross-section  of  the  embryo,  as  in 
Fig.  36,  B^  C.  The  mesoblastic  bands  are  at  lirst  solid,  but 
after  a  time  a  series  of  paired  cavities  appears  in  them,  con- 
tinually increasing  in  number  by  the  formation  of  new  cavities 
near  the  hinder  end  of  the  bands  as  they  increase  in  length.  \ 
cross-section  passing  through  one  pair  of  these  cavities  is  shown 
at  B^  Fig.  35.  As  the  bands  lengthen  they  also  extend  up- 
wards and  downwards  (6^,  Fig.  35),  until  finally  they  meet  al)ove 
and  below  the  archenteron.  The  cavities  at  the  sanio  time 
continue  to  increase  in  size,  and  finally  meet  above  and  below 
the  archenteron,  which  thus  becomes  surrounded  by  the  ImxIv- 
cavity  or  coelom  {D).  The  cavities  are  separated  by  the  double 
23artition-walls  of  mesoblast.  These  partitions  are  the  dissepi- 
ments, and  the  cavities  themselves  constitute  the  c(elom.  The 
outer  mesoblastic  Avail  of  each  cavitv  is  known  as  tha  so?ii at ic 
layer  (.§.??!.);  it  unites  with  the  ectoblast  to  constitute  the  body- 


86 


THE  BIOLOGY  OF  AN  ANIMAL. 


wall  (somatojdeure).  The  inner  wall,  or  splanchnic  layer 
{spl.?n),  unites  with  the  entoblast  to  constitute  the  wall  of  the 
alimentary  canal  (splanchnopleure).  An  ingrowth  of  ectoblast 
{stomod(BU7n)  takes  place  into  the  blastopore  to  form  the  pharynx, 
and  a  similar  ingrowth  at  the  opposite  extremity  {proctodceum) 
unites  with  the  blind  end  of  the  archenteron  to  form  the  anus 
and  terminal  part  of  the  intestine. 

As  to  its  origin,  therefore,  the  alimentary  canal  consists  of 
three   portions,    viz.  :    (1)    the    archenteron,    consisting   of   the 


B.VJU 

Fig.  38.— Diagram  of  a  cross-section  of  Lumhricus,  showing  tlie  relation  of  the 
various  organs,  etc.,  to  the  germ-layers.  Ectoblastic  structures  shaded  with  fine 
parallel  lines,  entoblastic  with  coarser  parallel  lines,  mesoblastic  with  cross-lines; 
a?.c,  alimentary  canals ;  ch,  chloragogue  layer ;  co?,  coelom ;  c.w,  circular  muscles 
of  body- wall;  c.ma,  circular  muscles  of  alimentary  wall;  ep,  lining  epithelium  of 
alimentary  canal;  d.r,  dorsal  vessel;  /ly,  hypodermis  or  skin;  l.m,  longitudinal 
muscles  of  body-wall;  l.m.a,  longitudinal  muscles  of  alimentary  wall;  ?i,  central 
part  of  nerve-cord ;  np,  nephridium ;  ns,  sheath  of  nerve-cord ;  p.e,  peritoneal 
epithelium ;  r,  reproductive  organs ;  s.i.v,  sub-intestinal  vessel. 

original  entoblast;  (2)  the  stomodgeum  or  pharyngeal  region, 
lined  by  ectoblast ;  and  (3)  the  proctodseum  or  hindmost  part, 
also  lined  by  ectoblast.  These  three  parts  are  called  the  fore- 
gut  (stomodaeum),  mid-gut  or  meUsenteron  (archenteron),  and 
hind-gut  (proctodseum),  and  it  is  a  remarkable  fact  that  these 
same  parts  can  be  distinguished  in  all  higher  animals,  not  ex- 
cepting man. 

The  body  now  becomes  jointed  by  the  appearance  of  trans- 
verse folds  opposite  the  dissepiments,  and  tlie  metamerism  of  the 
body  becomes  evident  on  the  exterior.  The  young  worm  has 
thus  reached  a  stage  (^,  Fig.  36)  where  its  resemblance  to  the 


FATE  OF  THE  GERM- LAYERS.  87 

adult  is  obvious.  It  has  an  elongated,  jointed  l)ody,  traversed 
by  the  alimentary  canal,  which  opens  in  front  by  the  mouth  and 
behind  by  the  anus.  The  metamerism  is  expressed  exterimlly 
by  tlie  jointed  appearance,  internally  l)y  the  presence  of  paired 
cavities  (coelom)  separated  by  dissepiments.  Both  the  body-wall 
and  the  alimentary  wall  consist  of  two  layers:  the  former  of 
ectoblast  without  and  somatic  mesoblast  within;  the  latter  of 
splanchnic  mesoblast  without  (i.e.,  towards  the  bodv-cavitv) 
and  either  entoblast  or  ectoblast  within,  according  as  we  con- 
sider the  mid-gut  on  the  one  hand,  or  the  fore-  and  hind-gut  on 
tlie  other.  This  is  shown  in  Fig.  38,  which  represents  a  cross- 
section  of  the  embryo  through  the  mid-gut.  If  this  be  clearly 
borne  in  mind  the  development  of  all  the  other  organs  is  easy  to 
understand,  since  they  are  formed  as  thickenings,  outgrowths, 
etc.,  of  the  parts  already  existing.  For  instance,  the  blood- 
vessels make  their  appearance  everywhere  throughout  the  meso- 
blast, and  the  reproductive  organs  are  at  first  mere  thickenings 
on  the  somatic  layer  of  the  mesoblast,  afterwards  separating 
more  or  less  from  it  so  as  to  lie  in  the  cavity  of  the  coelom. 
The  nervous  system  is  produced  by  thickenings  and  ingrowths 
from  the  ectoblast.  The  origin  of  the  different  parts  is  shown 
in  the  following  scheme  : — 

THE   GERM-LAYERS  AND  THEIR  DERIVATIVES. 


Ectoblast. 


Mesoblast. 


Entoblast. 


Outer  skin  (Hypodermis  and  Cuticle). 

Nerves  and  Ganglia. 

Lining  membrane  of  pharynx  (fore-gut). 

Lining  membrane  of  anus  and  hinder  part  of  intestine  (hind-gut). 


Muscles. 

Blood-vessels. 

Reproductive  organs,  , 

Outer  layers  of  alimentary  canal. 


Lining  membrane  of  greater  part  of  the  alimentary  canal  (mid-gut). 


The  above  statements*  as  to  the  origin  of  the  various  organs 
acquire  great  interest  in  view  of  the  fact  that  they  are  essen- 

*  Tlie  nepbridia  have  been  omitted  since  their  precise  origin  is  in  dispute. 
It  is  certain  that  tbe  outer  portion  of  the  tube  (inu.scular  part)  is  an  ingrowth 
from  the  ectoblast.  The  latest  researches  seem  to  show  that  the  entire  ne- 
phridium  has  the  same  origin,  though  some  autbors  describe  the  inner  portion 
as  arising  from  mesoblast. 


88  THE  BIOLOGY  OF  AN  ANIMAL. 

tiallj  true  of  all  animals  above  the  earthworm,  as  well  as  of 
many  below  it — of  all,  in  a  word,  in  which  the  three  germ- 
layers  are  developed,  i.e.,  all  those  above  the  Cmleiiterata^  or 
polyps,  jelly-fishes,  hydroids,  sponges,  etc.  In  man,  as  in  the 
earthworm  and  all  intermediate  forms,  the  ectoblast  gives  rise 
to  the  outer  skin  (epidermis),  the  brain  and  nerves,  fore-  and 
hind-gut ;  the  entoblast  gives  rise  to  the  lining  membrane  of  the 
stomach,  intestines,  and  other  parts  pertaining  to  the  mid-gut; 
while  the  somatic  and  splanchnic  layers  of  the  mesoblast  give 
rise  to  the  muscles,  kidneys,  reproductive  organs,  heart,  blood- 
vessels, etc.  It  is  now  generally  held  that  the  germ-layers 
throughout  the  animal  kingdom  (with  the  partial  exception  of 
the  Coelenterata  already  mentioned)  are  essentially  identical  in 
origin  and  fate.  This  view  is  known  as  the  Germ-layer  Theory. 
It  is  one  of  the  most  significant  and  important  generalizations 
which  the  study  of  Embryology  has  brought  to  light,  since  it 
recognizes  a  structural  identity  of  the  most  fundamental  kind 
among  all  the  higher  animals. 

Sooner  or  later  the  young  earthworm  bursts  through  the 
walls  of  the  capsule  and  makes  its  entry  into  the  world.  When 
first  hatched  it  is  about  an  inch  long  and  has  no  clitellum. 

It  is  a  curious  fact  that  in  certain  species  of  Lumbricus  the  young 
worms  are  almost  always  hatched  as  twins,  two  individuals  being  derived 
from  a  single  egg  by  a  process  which  is  described  by  Kleinenberg  in  the 
Quarterly  Journal  of  Microscopical  Science,  Vol,  XIX.,  1879.  It  often 
happens  that  the  twins  are  permanently  united  by  a  band  of  tissue,  as  in 
the  case  of  the  well-known  Siamese  twins. 

We  have  now  traced  roughly  the  evolution  of  a  complex 
many-celled  animal  from  a  suuple  one-celled  germ.  It  is  im- 
portant to  notice  at  this  point  a  few  general  principles  which  are 
true  of  higher  animals  in  general. 

1.  The  embryological  history  is  a  true  process  of  develop- 
ment,— not  a  mere  growth  or  unfolding  of  a  pre-existing  rudi- 
ment as  the  leaf  is  unfolded  from  the  bud.  ISTeither  the  ovum 
nor  any  of  the  earlier  stages  of  development  bears  the  slightest 
resemblance  to  an  earthworm.  The  embryo  undergoes  a  trans- 
formation of  structure  as  well  as  an  increase  of  size. 

2.  It  is  a  progress  from  a  one-celled  to  a  many-celled  con- 
dition. 


SUMMARY  OF  DEVELOPMENT.  89 

3.  It  is  a  progress  from  relative  simplicity  to  relative  eom- 
plexity.  The  ovmn  is  certainly  vastly  more  coinplcx  than  it 
appears  to  the  eye,  but  no  one  can  doubt  that  the  full-grown 
worm  is  more  complex  still. 

4.  It  is  a  progress  from  a  slightly  differentiated  to  a  liii^dily 
differentiated  condition.  The  life  of  the  ovum  is  that  of  a 
single  cell.  The  blastula  is  composed  of  a  number  of  nearly 
similar  cells,  which  in  the  gastrula  become  differentiated  into 
two  distinct  tissues.  In  later  stages  the  cells  become  differenti- 
ated into  many  different  tissues,  which  in  turn  build  up  different 
organs  performing  unlike  functions. 

5.  Lastly,  the  development  forms  a  cycle,  beginning  with 
the  germ-cell,  and  after  many  complicated  changes  resulting  in 
the  production  of  new  germ- cells,  which  repeat  the  process  and 
give  rise  to  a  new  generation.  All  other  cells  in  the  body  must 
sooner  or  later  die.  The  germ-cells  alone  persist  as  the  starting- 
point  to  which  the  cycle  of  life  continually  returns  (cf.  p.  78). 
Their  protoplasm,  the  ' '  gerin-])lasin^ ' '  is  the  bond  of  continuity 
that  links  together  the  successive  generations. 


CHAPTEK  YI. 

THE  BIOLOGY  OF  AN  ANIMAL   {Conti7\ued). 

The  Earthworm. 
Microscopic  Stkuctuke  or  Histology. 

"We  have  followed  the  develoj)nient  of  the  one-celled  germ 
through  a  stage,  the  hlastula^  in  which  it  consists  of  a  mass  of 
nearly  similar  cells  out  of  which  the  various  tissues  of  the  adult 
eventually  arise.  The  first  step  in  this  direction  is  the  differen- 
tiation of  the  germ-layers  or  three  primitive  tissues  (p.  84). 
As  the  embryo  develops,  the  cells  of  these  three  tissues  become 
differentiated  in  structure  to  fit  them  for  difl'erent  duties  in  the 
physiological  division  of  labor.  And  when  this  process  of  dif- 
ferentiation is  accomplished  and  the  adult  state  is  reached  we 
find  six  well-marked  varieties  of  tissue,  as  follows : — 

Principal  Tissues  of  Liimhricus, 

I.  EpitheliaL     Layer  of  cells  covering  free  surfaces. 

(a)  Pavement  Epithelium.     Cells  thin  and  flat,  arranged  like  the 

stones  of  a  pavement. 
(6)  Columnar  Epithelium.     Cells  elongated,  standing  side  by  side, 

palisade-like, 
(c)  Ciliated  Epithelium.     Columnar  or  cuboid,  and  bearing  cilia. 

II.  Muscular.  Cells  contractile  and  elongated  to  form  fibres.  Often 
arranged  in  parallel  masses  or  bundles. 

III.  Nervous.  Cells  pear-shaped  or  irregular,  with  large  nuclei ;  hav- 
ing processes  prolonged  into  slender  cords  or  fibres,  bundles  of  which  con- 
stitute the  nerves. 

IV.  Germinal.  Including  the  germ-cells.  At  first  in  the  form  of  epi- 
thelial cells  covering  the  ccelomic  surface,  but  afterwards  differentiated 
into  ova  and  spermatozoa. 

V.  Blood.  Isolated  cells  Of  corpuscles  floating  in  a  fluid  intercellular 
substance,  \h^  plasma. 

VI.  Connective  Tissue.  Cells  of  different  shapes,  often  branched  but 
sometimes  rounded,  separated  from  one  another  by  more  or  less  lifeless 
(intercellular)  substance  in  the  form  of  threads  or  homogeneous  material. 

90 


ARRANGEMENT  OF  TISSUES. 


91 


These  six  kinds  of  tissue  constitute  tlie  main  Imlk  of  the 
earthworm,  as  of  liigher  animals  generally ;  hut  there  are  \n  ad- 
dition other  tissues  which  will  be  treated  of  hereafter. 

Arrangement  of  the  Tissues.  The  sini})lest  and  most  diivct 
mode  of  discovering  the  arrangement  of  the  tissues  is  hv  the  nji- 
croscopical  study  of  thin  transverse  or  longitudinal  sections.      A 


am 


pe 


Ftg.  39.— Transverse  section  of  the  body  behind  the  clitellum.  a.(\  caN-ity  of  the  ali- 
mentary canal ;  c,  cuticle  ;  em,  ccelom  ;  cm,  circular  muscles ;  c.r,  cinular  vessel ; 
ci.u,  dorsal  vessel;  luj,  hypodermis;  l.m,  longitudinal  muscles;  ».r,  ventral  nerve- 
chain;  p.e,  peritoneal  epithelium;  s,  seta;  s.y,  setigerous  gland;  nA.v,  sub-intes- 
tinal vessel ;  s.w,  muscle  connecting  the  two  groups  of  setae  on  the  same  aide ;  f j/, 
typhlosole. 

transverse   section    taken  throuo^h    the    rei^ion  of   the   stomach- 
intestine    is   represented   in    Fig.    39.       Its    composition    is   as 
follows : — 
A.   Body -WALL. 

This  consists  of  five  layers,  viz.  (beginning  with  the  out- 
side),—  •* 

1.  Cuticle  ((?).  A  very  thin  transparent  nieml)rane.  not 
composed  of  cells  and  perforated  by  fine  pctres.  It  is  a  product 
or  secretion  of  the — 


92  THE  BIOLOGY  OF  AN  ANIMAL. 

2.  Hypodermis  {]nj)  (epidermis  or  sMn).  A  layer  of  colum- 
nar epithelium,  composed  of  several  kinds  of  elongated  cells,  set 
vertically  to  tlie  surface  of  the  body.  Some  of  these,  known  as 
gland-cells.^  have  the  power  of  producing  within  their  substance 
a  glairy  fluid  (mucus),  which  exudes  to  the  exterior  through  the 
pores  in  the  cuticle.  Others  (sensory  cells)  give  off  from  their 
inner  ends  nerve-iibres  which  may  be  traced  inwards  to  the 
ganglia  (Fig.  43). 

The  Clitellum  is  produced  by  an  enormous  thickening  of  the  hypoder 
mis,  caused  especially  by  a  great  development  of  the  gland-cells.  Three 
forms  of  these  may  be  distinguished,  which  probably  produce  different 
secretions.  The  tissue  is  permeated  by  numerous  minute  blood-vessels 
which  ramify  between  the  cells. 

3.  CirGidar  Ihiscles  {cm).  A  layer  of  parallel  muscle- 
libres  running  around  the  body.  On  the  upper  side  they  are 
intermingled  with  connective-tissue  cells  containing  a  granular 
brownish  substance  (pigment)  which  gives  to  the  dorsal  aspect 
its  darker  tint. 

4.  Longitudinal  Muscles  (l-.m).  A  layer  of  muscle-fibres 
running  lengthwise  of  the  body.  They  are  arranged  in  compli- 
cated bundles,  which  in  cross-sections  have  a  feathery  appear- 
ance. In  longitudinal  sections  they  appear  as  a  simple  layer,  and 
resemble  the  circular  fibres  as  seen  in  the  cross-section. 

The  circular  muscles  are  arranged  in  somewhat  similar  bun- 
dles, as  may  be  seen  in  longitudinal  sections. 

5.  Coelomic  or  Peritoneal  Epitlieliuin  {p.e.).  A  very  thin 
layer  of  flattened  cells  next  the  coelomic  cavdty. 

The  hypodermis,   and  therefore  also  the  cuticle  to  which  it 
gives  rise,  is  derived  from  the  ectoblast.     The  othei  layers  (3, 
4,  5)  arise  from  the  somatic  layer  of  the  mesoblast. 
B.   Alimentary  Canal. 

The  wall  of  this  tube  appears  in  cross-section  as  a  ring  sur- 
rounded by  the  coelom.  The  typhlosole  {ty)  is  seen  to  be  a  deep 
infolding  of  its  upper  portion.  In  the  middle  region  the  wall  is 
composed  of  five  layers  as  follows,  starting  from  the  alimentary 
cavity  (Fig.  40) : — 

1.  Lining  Epithelium  {ep).  A  layer  of  closely  packed,  nar- 
row ciliated  columnar  cells  with  oval  nuclei. 

2.  Vascular  Layer  {v. I).     Numerous  minute  blood-vessels. 


HISTOLOGY  OF  THE  ALIMENTARY  CANAL. 


93 


3.  Cii'cular  Muscles  (c.m).  A  thin  layer  uf  iimscle-fibres 
running  around  the  gut. 

4.  Longitudinal  Muscles  {J.m).  A  thin  layer  of  muscle- 
fibres  running  along  the  gut. 

5.  Chloragogue  Layer  (eh).  Composed  of  large  polyhedral 
or  rounded  cells  containing  yellowish-green  granules.  The  cells 
fill  the  hollow  of  the  typhlosole,  and  cover  the  surface  of  the 
dorsal  and  lateral  blood-vessels.  This  layer  represents  the 
splanchnic  part  of  the  peritoneal  epithelium. 

The  same  general  arrangemeut  exists  in  all  parts  of  the  alimentary 
canal,  but  is  sometimes  greatly  modified.  For  instance,  the  gizzard  and 
pharynx  are  lined  by  a  tough,  thick  cuticle,  and  the  muscular  layers  are 
enormously  developed.  In  a  part  of  the  gizzard  the  chloragogue-layer  is 
nearly  or  quite  absent  and  the  typlilosole  disappears.  A  fuller  description 
of  these  modifications  will  be  found  in  Brooks's  Handbook  of  Invertebrate 
Zoology.,  and  a  complete  account  in  Claparede,  Zeitschrift  far  wiasen- 
schaftliche  Zoologie,  Vol.  XIX.,  1869. 

The  lining  epithelium  is  derived  from  the  entoblast.  The 
remaining  layers  arise  by  differentiation  of  the  splanchnic  layer 
of  inesoblast. 


■■■■    r.'.  r..-    '.•■.•,■.■■    .*•:-:    'Ji         - 


■^•iii'->i^^'-^iy'*iij--  \^=iaiif    ij. 


r.  :■■■  :  ■. 


C.  //i. 


Fig.  40.— Highly  magnified  cross-section  through  tlio  wall  of  the  alimentary  cnnnl. 
c7i,  chloragogue  layer;  cm,  circular  muscles;  c.p,  lining  epithelium;  Lm,  longi- 
tudinal muscles ;  v.l,  vascular  layer. 

Blood-vessels  appear  in  the  section  as  rounded  or  irregular 
cavities  bounded  by  thin  walls.  They  consist  of  a  delicate  lining 
epithelium  covered  by  a  thin  layer  of  muscle-fibres.  In  the 
walls  of  the  stomach-intestine  the  vessels  are  often  completely 
invested  by  chloragogue-cells,   which  radiate    from  them   with 


94 


THE  BIOLOGY  OF  AN  ANIMAL. 


great  regularity  (Fig.  39).  The  liner  brandies  have  no  muscu- 
lar layer,  consisting  of  the  epithelium  alone. 

Lissepimeiits.  These  often  appear  in  cross  or  longitudinal 
sections.  They  consist  chiefly  of  muscle-flbres  irregularly  dis- 
posed, intermingled  with  connective-tissue  cells  and  fibres,  and 
covered  on  both  sides  with  the  peritoneal  epithelium. 

Nervous  System.  A  cross-section  of  a  ganglion  (Fig.  41) 
shows  it  to  be  comjDosed  of  two  distinct  parts,  viz. ,  (1)  the  gan- 


FiG.  41.— Highly  magnified  cross-section  of  a  ventral  ganglion,  g.f,  giant-fibres ;  Z.n, 
lateral  nerve ;  n.c,  nerve-cells ;  s,  muscular  sheath  of  the  ganglion ;  s.v,  sub-neu- 
ral vessel ;  s.n.i',  supra-neural  vessel. 

glion  proper  on  the  inside,  and  (2)  a  sheath  which  envelops  it. 
The  sheath  (^,  Fig.  41)  consists  of  two  layers,  viz.  : — 

1.  Peritoneal  Ejnthelium.     On  the  outside. 

2.  Muscular  Layer ^  or  sheath,  a  thick  layer  of  irregularly 
arranged  muscle-fibres  intermingled  with  connective  tissue.  Im- 
bedded in  it  are  the  sub-neural  blood-vessel  on  the  lower  side 
and  the  supra-neural  blood-vessels  on  each  side  above.  In  the 
middle  line  are  three  rounded  spaces  (^,  y.  Fig.  41),  which  are 
the  cross- sections  of  three  hollow  fibres  running  along  the  entire 
length  of  the  ventral  nerve-chain.  They  are  called  ' '  giant- 
fibres,  ' '  and  possibly  serve  to  support  the  soft  parts  of  the  nerve- 
cord. 

The  Ganglion  proper  is  distinctly  bilobed,  and  consists  of 
two  portions,  viz.  : — 

1.  Nerve-cells  {n.c),  I^umerous  pear-shaped  nerve-cells  near 
the  surface,  with  their  narrow  ends  turned  towards  the  centre, 
into  which  each  sends  a  single  branch  or  nerve-fibre.  They  are 
confined  chiefly  to  the  ventral  and  lateral  parts  of  the  ganglion. 


HISTOLOGY  OF  THE  N  Eli  VOL'S  SYSTEM. 


95 


2.  Flhwiis  Portion.  This  occupies  the  central  part.  It 
consists  of  a  close  and  complicated  network  of  nerve-libres  inter- 
mingled with  connective  tissue.  Some  of  these  li])res  communi- 
cate with  branches  of  the  nerve-cells,  as  stated  al«n'e;  otiiers 
run  out  into  the  lateral  nerves,  while  still  others  run  alunf*-  the 
connnissui'es  to  connect  with  libres  from  other  irantrlia 


Fig.  42.— Two  of  the  ventral  ganglia  (I,  II)  of  Lumhrkns  with  the  lateral  nerves, 
showing  some  of  the  motor  nerve-cells  and  fibres  (black),  a  sends  fibres  for- 
wards and  backwards  within  the  nerve-cord;  /»,  a  fibre  into  one  of  tlie  double 
nerves  on  its  own  side ;  c  and  d,  fibres  that  cross  to  the  nerves  of  the  opposite  side. 
(After  Retzius.) 

According  to  the  latest  researches  (of  Lenhossek  and  Retzius)  most  if 
not  all  of  the  nerve-cells  of  the  ventral  cord  are  motor  in  function.  Near 
the  centre  of  each  ganglion  (Fig.  42,  e)  in  a  sinirle  large  multipolar  cell  of 
doubtful  nature.  All  the  other  cells  are  either  bipolar  or  unipolar,  in  the 
latter  case  sending  out  a  single  branch  which  soon  divides  into  two.  In 
every  case  one  of  the  branches  breaks  up  into  fine  sub-divisions  within  the 
cord.  The  other  branch  in  most  cases  passes  out  of  the  cord  through  one 
of  the   lateral  nerves  to  the  muscles  or  other  peripheral  organs,  either 


96 


THE  BIOLOGY  OF  AN  ANIMAL, 


crossing  within  the  cord  to  the  opposite  side  of  the  body  or  making  exit 
on  its  own  side.  Some  of  the  cells,  however,  are  purely  "  commissural," 
i.e.,  neither  branch  leaves  the  cord. 

The  sensory  fibres  entering  from  the  periphery  terminate  freely  (not  in 
■nerve-cells),  breaking  up  into  numerous  fine  branches  on  the  same  side  of 
the  cord.     (Fig.  43.) 

The  nerves  leaving  the  central  system  are  mixed,  i.e.,  they  contain  both 
sensory  and  motor  fibres. 


nc 


Fig.  43.— Transverse  section  of  ventral  part  of  the  body,  showing  the  nervous  con- 
nections, n.c,  ventral  ganglion,  giving  off  a  lateral  nerve  at  l.n. ;  p.e.,  peritoneal 
epithelium;  l.m.,  longitudinal  muscles;  ?)i/,  hypodermis;  s,  seta.  A  single  motor 
nerve-cell  (black)  is  shown  sending  a  fibre  into  the  nerve  towards  the  left.  In 
the  nerve  to  the  right  are  sensory  fibres  proceeding  inward  from  the  sensory  cells 
(black)  of  the  hypodermis,  and  terminating  in  branching  extremities.  (After 
Lenhoss6k.) 

Sections  through  the  ventral  commissures  are  similar  to  those  through 
the  ganglia,  but  the  central  portion  (i.e.,  that  within  the  sheath)  is  smaller, 
is  divided  into  two  distinct  parts,  and  the  nerve-cells  are  less  abundant. 

Sections  through  the  nerves  show  them  to  consist  only  of  parallel  fibres 
surrounded  by  a  sheath  which  gradually  fades  away  as  the  nerves  grow 
smaller,  and  finally  disappears,  the  muscular  layer  first  disappearing,  and 
then  the  epithelial  covering. 

With  this  brief  sketch  of  the  histological  structure  of  the 
earthworm  we  conclude  our  morphological  stu(ij  of  the  animal. 
Those  who  desire  fuller  information  on  the  histology  will  find  a 
geneial  treatment  of  it  in  the  work  of  Claparede,  already  cited 
■at  p.  93.  Many  later  works  have  been  published  on  the  de- 
tailed histology. 


CHAPTER  YII. 

THE  BIOLOGY   OF  AN  ANIMAL  {Continued.) 
Physiology  of  the  Earthworm. 

In  the  preceding  pages  l)rief  descriptions  of  many  sjx-cial 
physiological  phenomena  have  l)een  given  in  connection  with  the 
detailed  descriptions  of  the  primary  functions  and  systems.  It 
now  remains  to  consider  the  more  general  problems  of  tlie  life  of 
the  animal,  and  especially  its  relations  to  the  enviromnent,  and 
the  transformations  of  matter  and  energy  which  it  elfects. 

The  Earthworm  and  its  Environment.  Tlie  earthworm  is  an 
organized  mass  of  living  matter  occupying  a  definite  position  in 
space  and  time,  and  existing  amid  certain  delinite  and  charactt-r- 
istic  physical  surroundings  which  constitute  its  ^'^  environment.'^ 

As  ordinarily  understood  the  term  environment  apjilies  only 
to  the  immediate  surroundings  of  the  animal — to  the  eartli 
through  which  it  burrows,  the  air  and  moisture  tliat  batlie  its 
surface,  and  the  like.  Strictly  s^^eaking,  however,  the  environ- 
ment includes  everything  that  may  in  any  manner  act  upon  the 
organism — that  is,  the  whole  universe  outside  the  worm.  For 
the  animal  is  directly  and  profoundly  affected  by  rays  of  light 
and  heat  that  travel  to  it  from  the  sun ;  it  is  extremely  sensitive 
to  the  alternations  of  day  and  niglit,  and  tlie  seasons  of  the  year; 
it  is  acted  on  by  gravity;  and  to  all  these,  as  well  as  to  more 
immediate  influences,  the  animal  makes  deflnite  responses. 

We  have  seen  that  the  body  of  the  eartliworm  is  a  compli- 
cated piece  of  mechanism  constructed  to  perform  certain  definite 
actions.  But  every  one  of  these  actions  is  in  one  way  or  an- 
other dependent  upon  the  environment  and  directly  or  indirectly 
relates  to  it.  At  every  moment  of  its  existence  the  organism  is 
acted  on  by  its  environment;  at  every  moment  it  reacts  upon 
the  environment,  maintaining  with  it  a  constantly  shifting  state 
of  equilibrium  which  finally  gives  way  only  when  the  life  of  the 
animal  draws  to  a  close. 

Adaptation  of  the  Organism  to  its  Environment.  In  its  rela- 
tions to  the  environment  the  earthworm  embodies  a  fundamental 

97 


98  THE  BIOLOGY  OF  AN  ANIMAL. 

biological  law,  viz. ,  that  the  living  organistn  must  he  adapted  to 
its  environment^  or,  in  otlier  words,  that  a  certain  harmony 
between  organism  and  envh'oninent  is  essential  to  the  continu- 
ance of  life,  and  any  influence  which  tends  to  disturb  or  destroy 
this  harmony  tends  to  disturb  or  destroy  life.  The  ada]3tation 
may  be  either  passive  (structural)  or  active  (functional).  Struc- 
tural adaptation  is  well  illustrated,  for  instance,  by  the  general 
shape  of  the  body,  so  well  adaj^ted  for  burrowing  through  the 
earth.  Again,  the  delicate  integument  gives  to  the  body  the 
flexibility  demanded  by  the  pecuKar  mode  of  locomotion;  it 
affords  at  the  same  time  a  highly  favorable  respiratory  surface — 
a  matter  of  no  small  importance  to  the  worm  in  its  badly-venti- 
lated burrow ;  and  yet  this  delicate  integument  does  not  lead  to 
desiccation,  because  the  animal  lives  alwavs  in  contact  with  moist 
earth.  The  alimentary  canal,  long  and  complicated,  is  most 
perfectly  fitted  for  working  over  and  extracting  nutriment  from 
the  earthy  diet.  The  reproductive  organs  are  a  remarkable  in- 
stance of  complex  structural  adaptation  in  an  animal  which  on 
the  whole  is  of  comparatively  simple  structure. 

Functional  adaptation  is  perhaps  best  shown  in  the  instinctive 
actions  or  "habits"  of  the  worm.  Its  nocturnal  mode  of  life 
(functional  adaptation  to  light)  and  its  ' '  timidity  ' '  protect  it 
from  heat,  desiccation,  from  birds  and  other  enemies.  In  win- 
ter or  in  seasons  of  drought  it  burrows  deep  into  the  earth. 

A  striking  instance  of  adaptation  is  shown  in  the  care  which 
is  taken  to  insure  the  welfare  of  the  embrvo  worms.      Minute, 

•y  7 

delicate,  and  helpless  as  they  are,  they  develop  in  safety  inside 
the  tough,  leathery  capsule  (p.  78),  floating  in  a  milklike 
liquid  which  is  at  once  their  cradle  and  their  food. 

Origin  of  Adaptations.  The  development  of  the  earthworm 
shows  that  its  whole  complex  bodily  mechanism  takes  origin  in  a 
single  cell  (p.  74),  and  that  all  the  remarkable  adaptations  ex- 
pressed in  its  structure  and  action  are  brought  about  by  a  gradual 
process  in  the  life-history  of  each  individual  worm.  There  is 
reason  to  believe  that  this  is  tj^^ical  of  the  ancestral  history  (de- 
scent) of  the  species  as  a  whole,  and  that  adaptation  has  been 
gradually  acquired  in  the  past.  We  know  that  environments 
change,  and  that  to  a  certain  extent  organisms  change  corre- 
spondingly through  functional  adaptation,  provided  the  change  of 


NUTRITION  OF  THE  AMMAL.  99 

environment  be  not  too  sudden  or  extrenic  In  utlicr  words 
the  organism  possesses  a  certain  i^lastlmtij  wliicli  enal)let^  it  to 
adapt  itself  to  graduallj-clianging  conditions  of  the  environment. 
Now  tliere  is  good  reason  to  believe  tliat  as  envirunmunt 
has  gradually  undergone  changes  in  tlie  })ast,  organisms  have 
gradually  undergone  corresponding  clianges  of  structure.  Those 
which  have  become  in  any  way  so  modilied  as  tu  he  most  per- 
fectly adapted  to  the  changed  environment  have  tended  to  sur- 
vive and  leave  similarly-adapted  descendants.  Those  wliich 
have  been  less  j)erfectlj  adapted  have  tended  to  die  out  tlirough 
lack  of  fitness  for  the  environment ;  and  by  this  process — called 
bj  Darwin  '' Natural  Selection"  and  by  Spencer  the  "Survival 
of  the  Fittest" — the  remarkable  adaptations  everywhere  met 
wdth  are  believed  to  have  been  gradually  worked  out. 

It  should  be  observed  that  Natural  Selection  does  not  really  explain  the 
origin  of  adaptations,  but  only  their  persistence  and  accumulation.  The 
theory  of  evolution  is  not  at  present  such  as  to  enable  us  to  say  with  cer- 
tainty what  causes  the  tirst  origin  of  adaptive  variations. 

Nutrition.  The  earthworm  does  work.  It  works  in  travel- 
ling about  and  in  forcing  its  way  through  the  soil ;  in  seizing, 
swallowing,  digesting,  and  absorbing  food ;  in  pumping  the 
blood;  in  maintaining  the  action  of  cilia;  in  receiving  and  send- 
ing out  nerve-impulses;  in  growing;  in  reproducing  itself — in 
short,  in  carrying  on  any  and  every  form  of  vital  action.  To 
live  is  to  work.  Now  work  involves  the  expenditure  of  energy, 
and  the  animal  body,  like  any  other  machine,  while  life  con- 
tinues, requires  a  continual  supply  of  energy.  It  is  clear  from 
what  has  been  said  on  p.  32  that  the  innnediate  source  of  tho 
energy  expended  in  vital  action  is  the  working  protoplasm  itsolf, 
which  undergoes  a  destructive  chemical  change  (kataholism  or 
destructive  metabolism)  having  the  nature  of  an  oxidation.  I'^i-nni 
this  it  follows  on  the  one  hanl  that  the  waste  ])ro(lucts  of  this 
action  must  be  ultimately  passed  out  of  the  body  ;us  excretions, 
and  on  the   other  hand  that  the  loss  must  ukimately  ]>e  made 

good  by  fresh  supplies  entering  the  animal  in  the  form  of  f 1. 

It  is  further  evident  that  the  income  must  ecpnU  the  (Uitgo  if  tiie 
animal  is  merely  to  hold  its  own,  and  must  exceed  it  if  the  ani- 
mal is  to  grow. 


100 


THE  BIOLOGY  OF  AN  ANIMAL. 


Thus  it  comes  about  tliat  there  is  a  more  or  less  steady  flow 
of  matter  and  of  energy  through  the  hving  organism,  which  is 
itself  a  centre  of  activity,  like  a  whirlpool  (p.  2).  The  chemical 
phenomena  accompanying  the  flow  of  matter  and  energy  through 
the  organism  are  those  of  nutrition  in  tlie  widest  sense.  This 
term  is  more  often  restricted  especially  to  the  phenomena  accom- 
panying the  income,  while  those  pertaining  to  the  outgo  are 
regarded  as  belonging  to  excretion.  The  intermediate  processes 
directly  connected  with  the  life  of  protoplasm  are  put  together 
under  the  head  of  metaholism  ^  they  include  both  the  construc- 
tive processes  by  which  protoplasm  is  built  uj)  {ayiabolism)  and 
the  destructive  processes  by  which  it  is  broken  down  (kataholmn) 
in  the  liberation  of  energy. 

Income.  It  is  difticult  to  determine  the  exact  income  of 
Lumhricus^  but  it  may  be  set  down  approximately  as  follows : — 

INCOME  OF  LUMBRICUS. 


Matter. 

Whence  Derived. 

1.  ProteMs. 

From  vegetal  or  animal  matters  taken  in  through  the  mouth. 

2.  Fats. 

From  vegetal  or  animal  matters  taken  in  through  the  mouth. 

3.  Carbohydrates. 

From  vegetal  or  animal  matters  taken  in  through  the  mouth. 

4.  Water. 

Taken  in  through  the  mouth,  or  perhaps  to  some  extent  ab- 
sorbed through  the  body- walls. 

5.  Free  oxygen. 

Absorbed  directly  from  the  atmosphere  or  ground-air  by  dif- 
fusion through  the  body-walls.  Sometimes  from  water  in 
which  it  is  dissolved. 

6.  Salts. 

Various  inorganic  salts  taken  along  with  other  food-stuffs. 

Energy. 

Potential. 

In  the  food. 

TJie  food-stuffs  are  converted  by  the  animal  into  the  sub- 
stance of  its  own  body  (protoplasm  and  all  its  derivatives),  and 
they  must  therefore  be  the  ultimate  source  of  energy.  It  fol- 
lows that  the  animal  takes  in  energy  only  in  the  potential  form 
(i.e.,  in  the  chemical  potential  between  the  oxidizable  proteids, 
carbohydrates  and  fats,  and  free  oxygen).     It  is  true  that  the 


DIGESTION  AND  ABSORPTION.  lOl 

animal  may  under  certain  circumstances  absorl)  kinetic  enerirs'  in 
the  form  of  heat,  but  this  is  available  only  as  a  condition^  not  as 
a  cause  of  protoplasmic  action.  In  this  inability  to  use  kinetic 
energy  the  earthworm  is  typical  of  animals  as  a  wIkjIc. 

Of  the  organic  portion  of  the  food  proteids  are  a  »iti£  qmv 
non^  and  in  this  respect  again  the  worm  is  a  type  of  aTiimal  life 
in  general.  Either  the  fats  or  the  carbohydrates  may  be  omitted 
(though  the  animal  probably  thrives  best  upon  a  mixed  diet  in 
which  both  are  present),  but  without  2)roteids  no  animal,  as  far 
as  is  known,  can  long  exist. 

General  History  of  the  Food.  Digestion  and  Absorption. 
LxiDibricus  takes  daily  into  its  alimentary  canal  a  certain  amount 
of  necessary  food-stuffs,  but  these  are  not  really  inside  the  body 
so  long  as  they  remain  in  the  alimentary  canal ;  for  this  is  shown 
by  its  development  to  be  only  a  part  of  the  outer  surface  folded 
in  to  afford  a  safe  receptacle  within  which  the  food  may  be 
worked  over.  Before  the  food  can  be  actuallv  taken  into  the 
body,  or  absorhed^  it  must  undergo  certain  chemical  changes  col- 
lectively called  digestion  (cf.  p.  49).  A  very  important  part 
of  this  process  consists  in  rendering  non-diffusible  substances  dif- 
fusible, in  order  that  they  may  pass  through  the  walls  of  the 
alimentary  canal  into  the  blood.  Proteids,  for  exam})le,  have 
been  shown  to  be  non-diffusible  (Chap.  III).  In  digestion  they 
are  changed  by  the  fluids  of  the  alimentary  canal  into  peptones 
— substances  much  like  proteids,  but  readily  diff"usible.  \\\ 
like  manner  the  non-diffusible  starch  is  chano^ed  into  diffusible 
sugar  and  becomes  capable  of  absorption.  It  is  higlily  j)robable 
that  all  carbohydrates  are  thus  turned  into  sugar.  The  fats  are 
probably  converted  in  part  into  soluble  and  diff"usi])le  soaps  whieh 
are  readily  absorbed,  but  are  mainly  enuilsitied  and  directly  passed 
into  ^tlie  cells  of  the  alimentary  tract  in  a  finely  divided  state. 
Nothino^,  however,  is  known  of  this  save  by  analoi^v  with  hii^her 
animals.  In  all  cases  digestion  takes  place  outside  the  ho(fy^  and 
is  only  preliminary  to  the  real  entrance  of  food  into  the  physit>- 
logical,  or  true,  interior. 

Metabolism.  After  absorption  into  the  body  proper  the 
incoming  matters  are  distributed  by  the  circulation  to  the  ulti- 
mate living  units  or  cells,  and  are  finally  taken  up  by  them  and 
built  into  their  substance.      There  is  reason  to  believe  that  each 


102  THE  BIOLOGY  OF  AN  ANIMAL. 

cell  takes  from  the  common  carrier,  the  blood,  only  such  ma- 
terials as  it  needs,  leading  a  somewhat  independent  life  as  to  its 
own  nutrition.  It  co-operates  with  other  cells  under  the  direc- 
tion of  the  nervous  system  (co-ordinating  mechanism),  but  to  a 
great  degree  is  independent  in  its  clioice  of  food — just  as  a  sol- 
dier in  a  well-fed  army  obeys  orders  for  the  common  good,  but 
yet  takes  only  what  he  chooses  from  the  daily  ration  supplied  to 
all. 

What  takes  place  within  the  cell  upon  the  entrance  of  the 
food  is  almost  wholly  unknown,  but  somehow  the  food-matters, 
rich  in  potential  energy,  are  built  up  into  the  living  substance 
probably  by  a  series  of  constructive  processes  culminating  in  pro- 
toplasm. Alongside  these  constructive  processes  (anabolism)  a 
contumal  destructive  action  goes  on  (katabolism) ;  for  living  mat- 
ter is  decomposed  and  energy  set  free  in  every  vital  action,  and 
vitality  or  life  is  a  continuous  process.  It  nmst  not  be  supposed, 
however,  that  either  the  synthetic  or  the  destructive  process  is  a 
single  act.  Both  probably  involve  long  and  complicated  chemi- 
cal transformations  but  the  precise  nature  of  these  changes  is  at 
present  almost  whqJly  unknown.  It  is  certain  that  the  destruc- 
tive action  is  in  a  general  way  a  process  of  oxidation  effected  by 
aid  of  the  free  oxygen  taken  in  in  respiration.  We  may  be 
sure,  however,  that  it  is  not  a  case  of  simple  combustion  (i.  e. ,  the 
protoplasm  is  not  "  burnt").  It  is  more  probably  analogous  to 
an  explosive  action,  the  oxygen  first  entering  into  a  loose  asso- 
ciation with  complex  organic  substances  in  the  protoplasm,  and 
then  suddenly  combining  with  them  under  the  appropriate  stim- 
ulus to  form  simpler  and  more  highly-oxidized  products.  Of 
the  precise  nature  of  the  process  we  are  quite  ignorant. 

Outgo.  Just  as  the  income  of  the  animal  represents  only  the 
first  term  in  a  series  of  constructive  processes,  so  the  outgo  is 
the  last  teiTu  of  a  series  of  destructive  actions  of  which  we  really 
know  very  little  save  through  their  results.  The  outgo  is  shown 
in  the  accompanying  table. 

Both  energy  and  matter  leave  the  cells,  and  finally  leave  the 
body — the  former  as  heat,  work  done,  or  energy  still  potential 
(in  urea  and  other  organic  matters);  the  latter  as  excretions, 
wliich  diffuse  freely  outwards  through  the  skin  and  nephridial 
surfaces. 


THE  ANIMAL  AND  ITS  ENVIRONMENT. 


\m 


OUTGO  OF  L^'^^nnIc^s:. 


Matter. 

Manner  oe  Exit. 

Carbon  dioxule  (COj). 

Mainly  by  diffusion  through  the  skin. 

Water  (HjO). 

Through  the  skin,  through  the  nephridia,  and  in  the  fwces. 

Urea  [(NH2)aC0],  and 
its  allies. 

Through  the  nephridia. 

Salts. 

Dissolved  in  the  water. 

Proteids     and    other 
organic  matters. 

In  the  substance  of  the  germ-cells,  the  egg-capsules,  and 
the  contained  nutrient  fluids. 

Energy. 

Potential. 

A  small  amount  still  remaining  in  urea,  in  the  germ-cells, 

etc. 

Kinetic. 

Work  performed.    Heat. 

Of  the  daily  outgo  tlie  water,  carbon  dioxide,  and  salts  are 
devoid  of  energy,  but  the  urea  contains  a  small  amount  wliicli  is 
a  sheer  loss  to  the  animal.  "Were  the  earthworm  a  perfect  ma- 
chine it  could  use  this  residue  of  energy  by  decomposing  the  urea 
into  simpler  compounds  [viz.,  ammonia  (Nil,),  carbon  dioxide 
(CO2),  and  water  (H^O)]  ;  but  it  lacks  this  power,  tliough  there 
are  certam  organisms  {Bacteria)  which  are  able  to  utilize  tlie  last 
traces  of  energy  in  urea  (p.  l!)7).  To  the  daily  outgo  must  be 
added  the  occasional  loss  both  of  matter  and  of  energy  sutfercd 
in  giving  rise  to  ova  and  spermatozoa,  and  in  providing  a  certain 
amount  of  food  and  protection  for  the  next  generation. 

Interaction  of  the  Animal  and  the  Environment.  The  action 
of  the  environment  upon  the  animal  has  already  been  sutHciently 
stated  (p.  97).  It  remains  to  point  out  the  changes  worked  by 
the  animal  on  the  environment.  These  changes  are  of  two 
kinds,  mechanical  (or  physical)  and  chemical.  The  most  imj)or- 
tant  of  the  former  is  the  continual  transformation  of  the  soil 
which  the  worms  effect,  as  Darwin  showed,  by  bringing  the 
deeper  layers  to  the  surface,  where  they  are  exposed  to  the  at- 
mosphere, and  also  by  dragging  superficial  objects  into  the  bur- 
rows.     The  chemical  changes  are  still  more  significant.      The 


104  THE  BIOLOGY  OF  AN  ANIMAL. 

general  effect  of  the  metabolism  of  the  animal  is  the  destruction 
by  oxidation  of  organic  matter ;  that  is,  matter  originally  taken 
from  the  environment  in  the  form  of  complex  proteids,  fats,  and 
carbohydrates  is  returned  to  it  in  the  form  of  sunpler  and  more 
highly  oxidized  substances,  of  which  the  most  important  are  car- 
bon dioxide  and  water  (both  inorganic  substances).  This  action 
furthermore  is  accompanied  by  a  dissipation  of  energy — that  is, 
a  conversion  of  potential  into  kinetic  energy. 

On  the  whole,  therefore,  the  action  of  the  animal  upon  the 
environment  is  that  of  an  oxidizing  agent,  a  reducer  of  comj^lex 
compounds  to  simpler  ones,  and  a  dissipator  of  energy.  And 
herein  it  is  typical  of  animals  in  general. 


CHAPTER   YIII. 

THE  BIOLOGY  OF  A  PLANT. 

The  Common  Brake  or  Fern. 

{Pteris  aquilina,  Linnaeus.) 

Foe  the  study  of  a  representative  vegetal  organism  some 
plant  should  be  chosen  which  may  be  readily  procured  and  is 
neither  very  high  nor  very  low  in  the  scale  of  organization. 
Such  a  plant  is  a  common  fern. 

Ferns  grow  generally  in  damp  and  shady  places,  tlioiigli 
they  are  by  no  means  confined  to  such  localities.  Some  of  tlie 
more  hardy  species  prefer  dry  rocks  or  even  bold  cliffs,  in  the 
crevices  of  which  they  find  support ;  others  live  in  open  tiekls 
or  forests,  and  still  others  on  sandy  hillsides.  In  tlie  northern 
United  States  there  are  altogether  some  fifty  species  of  wild 
ferns,  but  those  which  are  common  in  any  particuhir  locaHty  are 
seldom  more  than  a  score  in  number.  Throughout  the  \vh(>lc 
world  some  four  thousand  species  of  ferns  are  known,  l)ut  by 
far  the  greater  number  are  found  only  in  tropical  regions,  where 
the  climate  is  best  suited  to  their  wants.  At  an  earHer  j)eriod 
of  the  earth's  history  ferns  attained  a  great  size,  and  formed  a 
conspicuous  and  important  feature  of  the  vegetation.  At 
present,  however,  they  are  for  the  most  part  only  a  few  feet  in 
height.  Nearly  all  are  perennial;  that  is,  they  may  Hve  for  ;in 
indefinite  number  of  years.  Most  of  tliem  liave  creeping  or 
subterranean  stems;  but  some  of  the  troj)ical  species  have  eri'ct, 
aerial  stems,  sometimes  rising  to  a  height  of  fifty  feet  or  more 
and  forming  a  trunk  which  is  cylindrical,  of  equal  diametei 
throughout,  and  bears  leaves  only  at  the  sunnnit,  like  a  palm 
(tree-ferns). 

Of  all   the  ferns  perhaps  the  commonest  and   most   widely 

distributed  is  the  "  brake"  or  "  eagle-fern,"  which  is  known  to 

botanists  as  Pteris  aquilina^  Linnaeus,  or  rtevldlum  aquUinuni^ 

105 


106  THE  BIOLOGY  OF  A  PLANT. 

Kulin.  This  plant  is  not  only  common,  but  of  comparatively 
simple  structure ;  it  is  of  a  convenient  size,  and  lias  been  much 
studied.  It  may  therefore  be  taken  both  as  a  representative 
fern  and  as  a  representative  of  all  higher  vegetal  organisms. 

Habitat,  Name,  etc.  The  brake  occurs  widely  distributed  in 
the  United  States,  under  a  great  variety  of  conditions;  e.g.,  in 
loose  pine  groves,  especially  in  sandy  regions ;  in  open  wood- 
lands amongst  the  other  undergrowth ;  on  hillside  pastures  and 
in  thickets — indeed  almost  everywhere,  exce23t  in  very  wet  or 
very  dry  places.  It  appears  to  be  equally  common  elsewhere ; 
for,  according  to  Sir  W.  J.  Hooker,  Pteris  aquilina  grows 
' '  all  round  the  world,  both  within  the  tropics  and  in  the  north 
and  south  temperate  zones.  ...  In  Lapland  it  just  j^asses 
within  the  Arctic  circle,  ascending  in  Scotland  to  2000  feet, 
in  the  Cameroon  Mountains  to  7000  feet,  in  Abyssinia  to  8000 
or  9000  feet,  in  the  Himalayas  to  about  8000  feet."  {Sy7ioj)sis 
Filicuin?) 

"Pteris  {jirepL^^  the  common  Greek  name  iov  fern)^  signify- 
ing wing  or  feather,  well  accords  with  the  apj^earance  of  Pteris 
aquilina.,  the  most  common  and  most  generally  distributed  of 
European  ferns.  It  is  possible  that  this  fern  may  rank  as  the 
most  universally  distributed  of  all  vegetable  productions,  extend- 
ing its  dominion  from  west  to  east  over  continents  and  islands  in 
a  zone  reaching  from  Northern  Europe  and  Siberia  to  New 
Zealand,  where  it  is  represented  by,  and  perhaps  identical  with, 
the  well-known  Pteris  esculenta.  The  rhizome  of  our  plant 
like  that  of  the  latter  is  edible,  and  though  not  employed  in 
Great  Britain  as  food,  powdered  and  mixed  with  a  small  quan- 
tity of  barley-meal  it  is  niade  into  a  kind  of  gruei  called  gojio.^ 
in  use  among  the  poorer  inhabitants  of  the  Canary  Islands." — 
(Sowerby.) 

The  specific  name  aquilina  (ciquila^  eagle)  and  a  popular 
name,  "eagle-fern,"  in  Germany,  etc.,  have  come  from  a 
fanciful  likeness  of  the  dark  tissue  seen  in  a  transverse  section 
of  the  leaf-stalk  to  the  figure  of  an  outspread  eagle.  The  same 
figure  has,  however,  been  compared  to  an  oak-tree,  and  has  also 
given  rise  to  the  name  of  "  devil' s-foot  fern,"  from  its  alleged 
resemblance  to  "the  impression  of  the  deil's  foot,"  etc.,  etc. 

Tlie  popular  designation  of  this  plant  as  ' '  the  l)rake  ' '  testi- 


THE  PLANT  BODY.  107 

fies  to  its  great  abundance;   for  a  brake  is  a  dense  tliickct  (.r 
undergrowth — as  for  example  a  cane  '*  brake." 

When  fullj  grown  (P'ig.  44;  the  connnon  brake  has  a  leafy 
top  supported  by  a  polished,  dark-colored,  erect  stem,  whicli  in 
New  England  rises  to  a  height  of  from  one  to  four  feet  above 
the  ground.  In  this  climate,  however,  it  appears  to  be  some- 
what undersized,  for  it  grows  to  a  height  of  fourteen  feet  in 
the  Andes,*  and  in  Australia  attains  to  twice  the  height  of  a 
man,  forming  a  dense  undergrowth  beneath  tree-ferns  4n-l(n) 
feet  high.f  In  Great  Britain  it  is  from  six  inches  to  nine  feet 
high  (Sowerbj),  or  even  larger  in  exceptional  cases.  ''In  drv 
gravel  it  is  usually  present,  but  of  small  size;  while  in  tliick 
shady  woods  having  a  moist  and  rich  soil  it  attains  an  enormous 
size,  and  may  often  be  seen  climbing  up,  as  it  were,  among  the 
lower  branches  and  underwood,  resting  its  dehcate  pinnules 
on  the  little  twigs,  and  hanging  gracefully  over  them." 
(Newman.) 

General  Morphology  of  the  Body. 

The  body  of  the  fern,  like  that  of  the  earthworm,  consists 
of  cells,  grouped  to  form  tissues  and  organs.  Their  arrange- 
ment, however,  differs  widely  from  that  in  the  animal,  for  tlie 
plant-body  is  a  nearly  solid  mass,  and  there  are  no  extended 
internal  cavities  enclosing  internal  organs.  The  organs  of  the 
plant  are  for  the  most  part  external,  and  arise  by  local  modifica- 
tions of  the  general  mass.  Like  many  higher  plants  the  body 
of  the  fern  consists  of  an  axis  or  stem-bearing  branches,  from 
which  arise  leaves.  The  fern  differs  form  ordinary  trees,  how- 
ever, in  the  fact  that  the  stem,  with  its  branches,  lies  horiz(»ntal 
beneath  the  surface  of  the  ground.  Only  the  leaves  (fronds) 
rise  into  the  air.  (Fig.  44.)  It  is  convenient  to  describe  the 
body  of  the  brake,  accordingly,  as  consisting  of  two  very  (b*f- 
ferent  parts — one  green  and  leaHike,  which  rises  above  the 
ground;  the  other  black  and  rootlike,  lying  buried  in  the  soil. 
These  will  henceforth  be  spoken  of  as  the  aerial  and  the  muhr- 
groicnd  parts. 

The  underground  part  lies  at  a  depth  of  an  inch  to  a  foot 

*  Hooker,  I.  c. 

\  Kroue,  Botaii.  Jahrcshericht,  1876  (4),  346. 


Fig.  44.— The  Brake  (Pteris  aquiUna),  showing  part  of  the  underground  stem  (r.?J> 
and  two  leaves,  one  (?'),  of  the  present  year,  in  full  development;  the  other 
d'),  of  the  past  year,  dead  and  withered,  a.b,  apical  bud  at  the  extremity  of  a 
branch  which  bears  the  stumps  of  leaves  of  preceding  years  and  numerous- 
roots;  P,  mature  active  leaf  ;  P,  dead  leaf  of  preceding  year ;  l.m,  lamina  of  leaf  ; 
p,  pinna ;  r.h,  portion  of  main  rhizome ;  ;r,  younger  pinna,  which  is  shown  en- 


larged at  B.    This  pinna  is  nearly  similar  to  the  pinnules  of  older  pinnae. 


(Xi.> 


AERIAL  AND    UNDERGROUyD  PARTS.  109 

below  the  surface,  and  brandies  widely  in  various  directions. 
It  may  often  be  followed  for  a  loui^  distance,  and  in  snch  casfs. 
reveals  a  surprisingly  complicated  system  of  undori^round 
branches.  At  Urst  sight,  the  underground  porti(»n  of  the  fern 
appears  to  be  the  root,  but  a  closer  examination  shows  it  t»»  be 
really  the  stem  or  axis  of  the  plant,  which  ditfers  frcjm  ordinary 
stems  chiefly  in  the  fact  that  it  lies  horizontallv  uiidvr  the 
ground  instead  of  rising  vertically  above  it.  The  aerial  }K»rtion, 
which  is  often  taken  for  stem  and  leaf,  is  reallv  leaf  onlv.  The 
true  roots  are  the  line  fibres  which  spring  in  great  abundance 
from  the  underground  stem.  Underground  stems  more  or  less 
like  that  of  Pteris  are  not  uncommon — occurrini^,  for  iii>t;iiice, 
in  the  potato,  the  Solomon' s-seal,  the  onion,  etc.  In  /'A/'/.v, 
and  in  certain  other  cases,  the  underground  stem  is  technically 
called  the  rootstock  or  rhizome^  and  in  this  plant  it  constitntes. 
the  larger  and  more  persistent  part  of  the  organism.  In  the 
specimen  shown  in  Fig.  45  the  rhizome  was  about  eight  feet 
long  and  bore  two  leaves.  It  was  dug  out  of  sandy  soil  on  tlie 
edge  of  a  woodland,  and  lay  from  one  to  six  inches  below  the 
surface.  It  w^as  crossed  and  recrossed  in  all  directions,  both 
above  and  below,  by  the  rhizomes  of  its  neighbors,  the  whole 
constituting  a  coarse  network  of  underground  stems  loosely  till- 
ing the  upper  layer  of  the  soil. 

The  aerial  portion  {t\\Q  frond  or  leaf)  is  likewise  divisible 
into  a  number  of  parts,  comprising  in  the  first  place  the  h-af- 
stalk  or  stipe^  and  tlie  leaf  proper  or  lamina.  The  latter  is  suImH- 
vided  like  a  feather  {pinnately)  into  a  number  of  lobes  {j^'n/hBj 
Fig.  44),  which  vary  in  form  according  to  the  state  of  de- 
velopment of  the  leaf.  In  large  leaves  the  two  lower  i)inna^  are 
often  larger  than  the  others,  so  that  the  leaf  appears  to  consist 
of  three  principal  divisions,  and  is  said  to  be  "  t^'rfHftr  "  or  tri]» 
ly  divided  (Fig.  44,  A).  Each  pimia  is  in  tnrn  pinnately  sub- 
divided into  pinnules  (pinnuke)  or  leaflets  (Fig.  44,  />),  eadi  of 
which  is  traversed  down  the  middle  by  a  thickened  ridge  or 
rod,  the  midrib.  The  leaflets  sometimes  liave  smo»>th  «>ntlines, 
but  are  usually  lobed  along  tlie  edges,  as  in  Fig.  44.  />.  In 
this  case  their  form  is  said  to  be  pinnatifid.  Each  lohe  is  like- 
wise furnished  with  a  midrib.  The  stipe  enlarges  somewliat 
just   below  the  surface  of  the  ground,  then  grows  smaller  and 


110 


THE  BIOLOGY  OF  A  PLANT. 


joins  the  rliizome.  The  enlarge- 
ment is  of  considerable  interest, 
for  it  occurs  at  precisely  the 
point  of  greatest  strain  when  the 
leaf  is  bent  bj  the  wind  or  other- 
wise, and  must  serve  to  strength- 
en the  stipe. 

It  will  appear  from  the  fol- 
lowing description  that  the  plant 
body  exhibits  in  some  measure 
certain  general  forms  of  sym- 
metry and  differentiation  which 
in  a  broad  sense  may  be  regarded 
as  analogous  to  those  occurring  in 
the  animal.  The  rhizome  grows 
only  at  one  end,  and  in  its  struc- 
ture suggests  the  antero-posterior 
differentiation  of  the  animal.  It 
also  shows  a  slight  differentiation 
between  the  upper  and  lower 
surfaces,  which  appears  both  in 
the  external  form  and  in  the  ar- 
ran  element  of  the  internal  lines. 
It  is  furthermore  distinctly  bilat- 
eral, a  vertical  plane  dividing  it 
into  closely  similar  halves.  These 
features  are,  however,  far  less 
prominent  in  the  fern  than  in 
the  earthworm,  and  in  plants 
they  never  attain  a  high  degree 
of  development,  while  in  the 
higher  animals  they  are  among 
the  most  conspicuous  and  im- 
portant features  of  the  body. 
Fig  45— An  entire  C)f  morc  general  importance  in 
Sf'^the 'Tllv;s''?s  the    fern    is    the    repetition    of 

young     and    small,      •      m  i.        /i  i  x 

and  a  comparison  of  Similar    parts    (brauchcs,    roots, 

the  figure  wiUi  Fig,  ,  v         ,  .,  .  i  •   i 

44TN'iiishowsomeof  Icaves)    along   the    axis,    wnicii 

tliG  cliff crdicGS  Tdg- 

tween  leares  of  dif -  suffsiests,  pei'liaps,  a  Certain  an- 

ferentages.  ^^         '    "^  -^    ' 


AXIS  AND  APPENDAGES.  Ill 

aloo^y  to  animal  nietamerism,  tliouo^li  not  ii.>iiallv  recc-iiizt'd 
or  designated  by  the  same  term.  All  of  these  conditions  of 
differentiation  and  symmetry  are  more  easily  made  out  l)y  an 
examination  of  the  aerial  portion. 

The  plant  as  a  whole,  may  be  regarded  as  cousistin<'"  of 
an  axis  (the  rhizome  and  its  branches)  whifli  ]»oars  a  number 
of  appendages  in  the  form  of  roots  and  lea\'es.  The  axis  fonus 
the  central  body  or  trunk  of  the  plant,  and  in  it  m<jst  of  its  mat- 
ter and  energy  are  stored;  the  appendages  are  organs  for  takiu"- 
in  food,  for  excretion,  for  respiration,  for  repro<luction,  etc. 

The  Underground  Stem,  or  Rhizome,  and  its  Branches.  The 
rhizome  is  a  hard  black,  elongated,  and  branching  stem,  gener- 
ally flattened  somewhat  in  the  vertical  direction  as  it  lies  in  the 
earth,  and  expanded  slightly  on  either  side  to  form  well-marked 
lateral  folds — the  lateral  ridges.  Its  thickness  is  seldom  more 
than  half  an  inch,  and  usually  considerably  less.  In  transverse 
section  it  has  the  outline  shown  in  Fig.  48,  and  the  marginal 
part  only  is  black.  The  branches  repeat  in  all  respects  the  form 
and  structure  of  the  main  axis.  Both  the  main  axis  and  the 
branches  end  either  in  conical,  pointed,  and  fleshy  structures 
about  two  inches  long,  or  in  blunt,  yellowish  knol)s,  ])lainly  de- 
pressed in  the  centre.  At  these  ends  tlie  rhizome  grows ;  hence 
they  are  called  the  growing  points  or  apical  huds  (Figs.  44,  47). 
Besides  the  apical  buds  the  rhizome  bears  nearly  always  one 
or  more  dead,  decaying  tips.  These  arise  in  the  f(^llowing  man- 
ner:  After  attaining  a  certain  length  both  the  rhizome  and  its 
branches  gradually  die  away  behind.  Death  of  the  hinder  part 
follows  at  about  the  same  rate  with  which  growth  advances  at 
the  apical  buds;  so  that  the  total  length  may  not  change  mate- 
rially from  year  to  year.  It  is  ob^'ious  that  this  {)rocess  must 
result  in  the  gradual  and  successive  detachment  of  the  brandies 
from  the  main  axis.  Each  branch,  now  become  an  inde]>end- 
ent  rhizome,  repeats  the  process;  and  in  this  manner  a  single 
original  rhizome  may  give  rise  to  large  numbers  of  distinct 
plants,  all  of  which  have  been  at  some  time  in  material  connec- 
tion with  an  ancestral  stock.  This  process  is  evidently  a  kind  of 
reproduction  (though  it  is  not  the  most  important  or  most  obvi- 
ous means  for  the  propagation  of  the  plant),  and  in  this  way  a 
large  area  may  be  occupied  by  distinct,  though  related,  plants 


112 


THE  BIOLOGY  OF  A  PLAN  I. 


whose  brandling  rhizomes  cross  and  recross,  making  the  subter- 
ranean network  ah-eady  described,  p.  109. 

Origin  of  Leaves  upon  the  Rhizome  and  its  Branches.  The 
young  plant  of  Pteris  puts  up  a  number  of  leaves  (7-12)  yearly, 
but  the  adult  generally  develops  one  only,  which  grows  very 
slowly,  requiring  two  years  before  it  unfolds.  Towards  the  end 
of  the  first  year  it  is  recognizable  only  as  a  mmute  knob  at  the 
bottom  of  a  depression  near  the  growing  point.  At  the  begin- 
ning of  the  second  year  it  is  perhaps  an  inch  high,  the  stalk 


Fig.  46.  (After  Sachs.)— Developing  leaf,  etc.,  of  Pteris.  A,  end  of  a  branch  show- 
ing the  apical  bud  and  the  rudiment  of  a  leaf;  B,  a  rudimentary  leaf ;  C,a 
similar  leaf  in  longitudinal  section,  showing  the  infolded  lamina  (?),  the  attach- 
ment to  the  rhizome,  and  the  prolongation  of  the  tissues  of  the  latter  into  the 
leaf ;  D,  lamina  of  a  very  young  leaf ;  E,  horizontal  section  through  a  growing 
point  which  has  just  forked  to  form  two  apical  buds.  a.h.  apical  bud;  ep,  epi- 
dermis and  underlying  sclerotic  parenchyma ;  f.b,  fibro- vascular  bundles ;  Z, 
lamina ;  r,  root ;  s.p,  sclerotic  prosenchyma ;  a:,  an  adventitious  bud  at  the  base 
of  the  leaf. 

only  having  appeared.  At  the  end  of  the  second  year  the  lamina 
is  developed,  and  hangs  down  as  shown  in  Fig.  46,  0.  Early  in 
the  spring  of  the  third  year  it  breaks  through  the  ground,  and 
grows  rapidly  to  the  fully-matured  state. 


LEAVES  AND  lilllZOME. 


113 


The  leaves  usually  arise  near  tlie  apical  l)U(ls  of  the  main 
axis  or  of  the  branches.  Behind  eacli  mature  leaf  remnants  of 
the  leaves  of  preceding  years  are  often  to  he  found,  altfrnatinj; 
on  tlie  sides  of  the  rhizome  in  reguhir  successicjn,  and  showing 
various  stages  of  decay.  The  iirst  of  these  (whicli  is  on  the 
opposite  side  of  the  rhizome  from  the  hving  leaf)  was  aUve  the 
previous  year;  the  next  (on  the  same  side  with  the  hving  Icafi  is 
the  leaf  of  the  year  before  that;  and  s(j  on.  Fig.  47  sIkhvs  an 
example  of  this  sort.     The  leaf  of  the  present  year,  l\  is  fully 


Fig.  47.  (After  Sachs.)— Branch  of  a  rhizome  of  Pteri.%  showing  the  apical 
the  stumps  of  a  number  of  successive  leaves  (?',  P,  P,  etc.),  and  a  part  of 
rnizome  (rh),   r,  root. 


bud  (a.h\ 
tho  main 


developed;  and  the  relics  of  tlie  leaves  of  the  ])receding  years 
are  indicated  at  l\  1%  etc.  ;   r  is  the  rudiment  of  next  year's  leaf. 

Internal  Structure  of  the  Rhizome.  The  rhizome  is  a  nearly 
solid  mass,  consisting:  of  manv  different  kinds  of  cells,  united 
into  different  tissues,  and  having  a  very  complicated  arrange- 
ment. Its  study  is  somewhat  difficnlt.  Nevertheless  the  ar- 
rangement of  the  cells  is  definite  and  constant,  and  merits  careful 
attention,  since  it  has  many  features  which  are  characteristic  <»f 
the  cellular  structure  of  the  stems  of  higher  ])lants.  AVe  shall 
first  examine  its  more  obvious  anatomv  as  dis])lave<l  in  transverse 
and  longitudinal  sections,  afterwards  making  a  careful  micro- 
scopical study  of  the  cells  and  tissues. 

Seen  with  a  hand-lens  or  tlie  naked  eye,  a  transverse  section 
of  the  rhizome  (Fig.   48)  presents  a  white   or  yellowish   hack- 


114 


THE  BIOLOGY  OF  A  PLANT. 


pround  bounded  bj  a  black  margin  (the  epidermii)  and  marked 
by  various  colored  or  pale  spots  and  bands ;  the  latter  are  differ- 
ent tissues,  or  systems  of  tissue.  These  different  structures  are 
arranged  in  three  groups  or  systems  of  tissue^  which  are  found 


spro 


Fig.  48.— Cross-section  of  the  rhizome  of  Pteris.  Lr,  lateral  ridges;  /.p,  fundamental 
parenchyma;  s.p,  sclerotic  parenchyma;  s.pro,  sclerotic  prosenchyma;  /.b,  x 
flbro-vascular  bundles. 

among  all  higher  plants  in  essentially  the  same  form,  though 
differing  widely  in  the  minor  details  of  their  arrangement. 
These  are : — 

I.   The  Fundamental  System  of  Tissues. 
II.   The  Epidermal  System. 
III.   The  Fibro-vascular  System. 

The  Fundmnental  system  consists  in  Pteris  of  three  tissues : 

(ci)  fimdainental parenchyma  (Fig.  4:8,y.j9),  the  soft  whitish 
mass  forming  the  principal  substance  of  the  rhizome ; 

(5)  sclerotic  parenchym^a  (s.p),  the  brown  hard  tissue  lying 
just  below  the  epidermis,  from  which  it  is  scarcely  distinguish- 
able; 

(c)  sclerotic  prosencJiym^a  (s.pro),  black  or  reddish  dots  and 
bands  of  extremely  hard  tissue,  most  of  which  is  contained  in  two 
conspicuous  bands  lying  one  on  either  side  of  a  plane  joining 
the  lateral  ridges. 


TUE  GREAT  TISSUE-SYSTEMS.  115 

Tlie  sclerotic  pareiiclivnia  and  the  sclerotic  i^rosencliviiiu  Ijutli 
arise  through  a  transforniatioii  (hardening,  ctc.j  of  portions  of 
originally-soft  fundamental  parenchyma.  In  most  ])lants  ahove 
the  ferns  the  fundamental  system  contains  neither  of  these  tissues. 

The  Flhro-vascular  system  is  composed  of  loiitrituchnal 
threads  or  strands  of  tissue  known  as  i\nd  jihro-vascular  hum/les^ 
and  these  in  one  form  or  another  are  characteristic  of  all  hiirhcr 
plants.  They  appear  here  and  there  in  the  section  (Fig.  48,/'.^) 
as  indistinct,  pale  or  silvery  areas  of  a  roundish,  oval,  or  elon- 
gated shape.  Closely  examined  they  show  an  open  texture,  un- 
closing spaces  which  are  sections  of  empty  tubes,  or  vessels  and 
fibres,  from  which  the  .bundles  take  their  name. 

The  Epidermal  system  consists  of  a  single  tissue,  the  epider- 
mis^ which  covers  the  outside  of  the  rhizome. 

By  a  simple  dissection  of  the  stem  with  a  knife  the  sclerotic 
prosenchyma  and  the  fibro -vascular  bundles  may  be  seen  to  be 
long  strands  or  bands,  coursing  through  the  softer  fundamental 
tissues.   • 

It  should  be  clearly  understood  that  these  three  systems  are, 
in  general,  not  single  tissues,  but  groups  of  tissues  which  are 
constantly  associated  together  for  the  performance  of  certain 
functions."^ 

Microscopic  Anato:my  (Histology)  of  the  Ehizome. 

General  Account.  Microscopic  study  of  thin  sections  of  the 
rhizome  shows  the  various  tissues  to  be  composed  of  inmimerable 
closely-crowded  cells,  which  dilfer  very  widely  in  structure  and 
in  function.  In  studying  these  cells  the  student  should  not  lose 
sight  of  the  fact  that  they  are  objects  having  three  dimeusions, 
of  which  only  two  are  seen  in  sections.  And  hence  a  siugk'  sec- 
tion may  give  an  imperfect  or  entirely  false  impression  of  the 
real  form  of  the  cells, — just  as  the  faceof  a  wall  of  ma^^oury  may 
give  only  an  imperfect  idea  of  the  blocks  of  which   it   is  built. 


*  This  classification  of  the  tissues  is  only  a  matter  of  convenience,  and  ha* 
little  scientific  value.  B\^  many  botanists  it  has  been  rejected  alto^«'ther  ;  but 
no  apology  for  its  use  need  be  made  by  those  who.  like  the  authors,  have 
found  it  useful,  so  long  as  it  is  defended  by  Sachs  (\v)io  first  introduced  it)  and 
its  value  for  beginners  is  conceded  by  De  Bary. 


116 


THE  BIO  LOO  Y  OF  A  PLANT. 


Por  tliis  reason  many  of  tlie  cells  cf^n  only  be  understood  by  a 
■comparison  of  transverse  and  longitudinal  sections,  and  these 
should  be  studied  together  until  their  relations  are  thoroughly 
mastered. 

The  following  table  gives  brief  definitions  of  the  leading 
vegetal  tissues  and  is  good  not  only  for  Pteris  but  for  all 
plants : — 

PRINCIPAL  ADULT  VEGETAL  TISSUES. 


Tissues. 

Characteristics. 

1.  Epidermia. 

Cells  in  a  single  layer  covering  the  outer  surface. 

2.  Pai'enchyma. 

Masses  of  ceUs,  rounded,  prismatic  or  polyhedral,  usually  incom- 
pletely joined  at  the  angles,  thus  leaving  intercellular  spaces. 
Not  much  longer  than  broad.    Thin-walled. 

3.  Prosenchyma. 

Cells  elongated,  typically  massed,  without  intercellular  spaces. 

4.  Sieve-tubes. 

Cells  elongated,  thin-walled,  panelled  with  perforated  areas, 
containing  proteids. 

5.  Ti^acheids. 

CeUs  thick-walled,  elongated,  pointed,  hard ;  walls  pitted ;  filled 
with  air. 

6.  Trachece     or 
vessels. 

Cells  very  slender,  elongated,  opening  into  one  another  at  their 
ends,  often  spirally  thickened,  and  filled  with  air. 

These  six  tissues  are  not  only  found  in  the  rhizome,  but  ex- 
tend throughout  the  roots  and  the  fronds  as  well.  Moreover, 
all  the  tissues  not  only  of  the  fern  but  of  all  higher  plants  are 
varieties  of  them. 

Special  Account.  It  must  not  be  forgotten  that  the  differences 
between  tissues  are  only  the  outcome  of  the  differences  between 
their  component  cells  (p.  13).  So  that  the  study  of  the  histology 
of  the  rhizome,  even  if  preceded  (as  it  may  well  be)  by  a  dissec- 
tion, and  a  naked-eye  examination  of  some  of  the  tissues,  event- 
ually resolves  itself  into  the  careful  microscopic  study  of  the 
several  kinds  of  cells  composing  those  tissues. 

The  mature  parts  of  the  rhizome  contain  at  least  nine  very 
different  kinds  of  cells,  the  characteristics  and  grouping  of 
w^hicli  are  shown  in  the  following  table.  In  the  apical  buds, 
liowever,  this  arrangement  disappears,  and  all  the  cells  appear 
-closely  similar. 


HISTOLOGY  OF  THE  lilllZOME.  117 

MINUTE  ANATOMY  OF  THE   RHIZOME  OF  PTKIUS  AQUILINA. 


System. 


I.  Epidermal 


Tissues. 


1.  Epidermis. 


II.  Funda- 
mental, 


III.  Fibro- 
vascular. 


3.  Fundamental 
parenchyma. 


CHAUACTEUISTK.S. 


Cells    polytronal    in    crftss-softion,  cinijfv.    Walls 
hard,  tliickened,  especially  towards  the  outsiil.- 
(Fig.  49.) 


Cells  rounded  or  poly^ronal  in  fross-sortion.  color- 
less.  Thin-wallcd,  coiitniiiinu  prutopluMn,  im. 
ck'us  and  starch.     Iiitercellulur  spaces  prescrn 

(Fig.  5:-', /.p.) 


•^olprntin     Ttnr  Cells  polygonal  or  semi-fusiform  in  section,  ncarlv 
*     tSumn  empty.    No    interc-ellular    spaces.     Walls    har-l 

cncnyma.  ^^^  brown,  thickened.     (Fig.  49.) 


4.  Sclerotic   pios- 

enchyma     (or  Cells  fusiform,  empty.    "Walls  thick,  red.    (Fig.  5(1.  i 
sclerenchyma) 


5.  Wood  -  par  en 
chyma. 


Like  the  fundamental  parenchyma,  but  with  more 
elongated  cells.    (Figs.  52,  .53.) 


6.  Phloem-paren- 
chyma. 


7.  Phloem-prosen- 
chyma,  or 
bast-fihres. 


8.  Sieve-tubes. 


9.  Tracheids  (lad- 
der-cells). 


10.  Trachece  or  ves- 
sels (spiral). 


Precisely  like  5,  differing  only  in  position. 


Cells  fusiform,  rich  in  protoplasm,  colorless.   Walls 
thick,  soft.     (Figs.  52,  53.) 


Having   the   ordinary  characters   (see  preceding 
table).     (Figs.  52-5t.) 


Pits  transversely  elongated  (scalariform).    (Figs. 
53-53.) 


Very  slender,  with  one  or  two  internal  spiral  thick- 
enings.   (Fig.  53.) 


Besides  the  above-mentioned  tissues,  the  rliizonie  contains 
certain  other  secondary  varieties  whicli  will  be  descri])cd  further 
on. 

Epidermal  System.  Epidermis.  It  is  the  fuiu'tiou  <>f  the 
epidermis  (aided  in  this  case  by  the  underlying  sclerotic  ])arc'n- 
chyma)  to  protect  the  inner  tissues  from  contact  with  thi'  soil 
and  to  guard  against  desiccation  of  the  rhizome  during  ch-oiights. 
The  cells  (Fig.  49)  are  dead  and  empty,  with  enormously  thick, 
hard  walls  perforated  by  numerous  branching  canals.  The  outer 
wall  is  especially  thick. 

Fundamental  System.  The  tissues  of  this  system  form  the 
main  body  of  the  plant,  and  in  the  fern  have  two  widely  ditTcr- 


118 


THE  BIOLOGY  OF  A  PLANT. 


Fro.  49.— Section  showing  the  epidermis  (ep)  and  the  underlying  sclerotic  paren- 
chyma (.s.p)  of  the  rhizome  of  Pteris  aquiUna.  Canals,  sometimes  branching,  are 
everywhere  seen.  These  served  to  keep  the  once-living  cells  in  material  con- 
nection. 


Fig.  50.— Cross-section  of  sclerotic  prosenchyma  of  the  rhizome  of  Pteris  aquilina. 
The  enormously  thickened  walls  consist  of  three  layers,  are  perforated  by  canals, 
and  are  lignified  or  turned  into  wood. 


HISTOLOGY  OF  THE  RHIZOME. 


119 


ent  functions.  T\\q  fu7ida7ne7ital parenchytna  is  a  kind  of  store- 
house in  wliicli  matter  and  energy  are  stored — inainlv  in  tlie 
form  of  starch,  CoHio^^b — ^^^^^  in  wliich  active  chemical  cliani::e8 
take  place.  The  cells  are  thin-walled  and  soft,  and  are  ratln-r 
loosely  joined  together,  leaving  numerous  intercellular  spaces 
(Figs.  52,  53).  They  contain  protoplasm  and  a  nuck'us,  and 
very  numerous  rounded  grains  of  starch.  This  starch  is  sturud 
up  by  the  plant  during  the  summer  as  a  reserve  supj)ly  of  fo(»d 
— just  as  hibernating  animals  store  up  fat  in  tlieir  1)odies  for  use 
during  the  winter.  Accordingly,  starch  increases  in  (juantity 
during  the  summer  and  decreases  in  the  spring  when  the  j)laiit 
resumes  its  growth,  before  the  leaves  are  unfolded.  The  j)aren- 
chy ma  probably  has  also  the  function  of  conducting  various  sub- 
stances (especially  dissolved  sugar)  through  the  plant  by  diffusion 
from  cell  to  cell. 


The  sclerotic  parenchyma  and  sclerotic  prosenchyma  (Figs.  49,  50)  are 
dead,  and  hence  play  a  passive  part  in  the  adult  vegetal  economy.  The 
former  co-operates  with  the  epidermis  ;  the 
latter  probably  serves  in  part  to  support  the 
soft  tissues,  and  to  some  extent  affords  a 
channel  for  the  conveyance  of  the  sap.  The 
sap,  however,  does  not  flow  through  the 
cavities,  but  passes  slowly  along  the  sub- 
stance of  the  porous  walls.  The  cells  of 
both  these  sclerotic  tissues  have  very  thick, 
hard,  brown  walls,  perforated  here  and 
there  by  narrow  canals.  The  cells  of  the 
parenchyma  are  prismatic  or  polyhedral ; 
those  of  the  prosenchyma  elongated,  and 
pointed  at  their  ends.  In  both,  the  proto- 
plasm and  nuclei  disappear  when  the  cells 
are  fully  formed.  Towards  the  apical  buds 
both  fade  into  ordinary  fundamental  paren- 
chyma. 

Fibro-vascular  System.     The  Jihro- 
vascular    bundles  (p.  115)  are    long  pio.  m.   (Aft.^rSarhs.)-viow  of 
strands  or  bands  of  tissue  which  ap-     ^he  rhizome,  -'7'; /^/[^J'^T;^ 

*■         to  be  transparent  so  as  to  snow 
pear    in    cross-section   as  isolated  spots      the  network  of  tlie  upper  nbro- 

aT  4  o\  rm  1  n  „  ,,  ^i.      vasciihir  biiiunes.    /,  a  leaf, 

ng.    48).       The    bundles    are    not 

really  isolated,  how^ever,  but  join  one  another  here  and   there, 

forming  an  open  network  (Fig.  51),  which  can  only  be  seen  in  a 


120 


THE  BIOLOGY  OF  A   PLANT. 


lateral  view  of  tlie  rliizome.  From  this  network  bundles  are 
given  off  which  extend  on  the  one  hand  into  the  roots  and  on 
the  other  into  the  leaves,  branching  in  the  latter  to  form  tlie 
complicated  system  of  veins  to  be  described  hereafter  (p.  129). 

Eacli  bundle  consists  of  a  number  of  different  tissues  which, 
broadly  speaking,  have  the  function  of  conducting  sap  from  one 
part  of  the  plant  to  another. 


f  IG.  52.— Highly  magnified  cross-section  of  a  fibro-vascular  bundle  surround-id  by 
the  fundamental  parenchyma,  /.p.  t,  scalariform  tracheids  ;  h.s,  bundle-sheath ; 
p.s,  phloem-sheath  ;  ?>./,  bast-fibres ;  s.t,  sieve-tubes ;  p.p,  phloem-parenchyma ; 
u\p,  wood  (xylem)  parenchyma;  s.i%  spiral  vessel. 

These  tissues  have  the  following  definite  arrangement.  Beginning  with 
the  outside  of  a  bundle,  we  find  (Figs.  52,  53) — 

1.  Bundle- sheath ;  a  single  layer  of  elongated  cells  enveloping  the 
bundle,  probably  derived  from  and  belonging  to  the  fundamental  system. 

2.  Phloem-sheath  ;  a  single  layer  of  larger  parenchymatous  cells  con- 
taining starch  in  large  quantities. 

3.  Bast-fibres ;  soft,  thick-walled,  elongated,  pointed  cells  containing 
protoplasm  and  large  nuclei. 

4.  Sieve-tubes;  larger,  soft,  thin-walled,  elongated  cells  containing 
protoplasm  and  having  the  walls  marked  by  areas  perforated  by  numerous 
fine  pores' (panelled).  They  join  at  the  ends  by  oblique  panelled  partitions 
(shown  in  Figs.  52  and  53). 


HISTOLOGY  OF  TUE  RHIZOME. 


121 


^.Phloem-parenchyma;  ordinary  parenchymatous  cells  filled  with 
starch,  scattered  here  and  there  among  the  bast-fibres  and  sieve-tuljcs. 

6.  Tracheids  {scalariform)  or  "ladder-cells"  ;  occupying  most  of  the 
central  part  of  the  bundle.  Their  structure  calls  for  some  remark.  They 
are  empty  or  air-filled  fusiform  tubes,  whose  hard,  thick  walls  are  in  the 
young  tissue  sculptured  with  great  regularity  into  a  series  of  transverse 
hollows  or  pits,  which  finally  become  actual  holes.  The  walls  of  the 
tracheid  are  therefore  continuous  at  the  angles,  but  along  their  plane  sur- 


ps.     s.t 


fp.     1.8. 


Fig.  53.— Longitudinal  section  of  a  fibro-vascular  bundle,  surrounded  by  the  fun- 
damental parenchyma.  />./,  bast-flbres;  7>.s,  bundle-sheath;  /.p,  fundauiental 
parenchma ;  p.p,  phloem-parenchyma;  y.f^.,  phloem-sheath;  s.f,*  sieve-tubes;  f, 
scalariform  tracheids  or  ladder-cells  ;  w.i>.,  wood-parenchyma. 


faces  become  converted  into  a  series  of  parallel  bars,  makinc:  a  grating  of 
singular  beauty.  The  slits  between  the  bars  are  not  rectangular  jiassages 
through  the  w^all,  but  are  rather  like  elongated,  flattened  funnels,  opening 
outwards.  The  sides  of  the  funnels  are  called  the  borders  of  the  pits:  and 
pits  of  this  sort  are  called  bordered  scalariform  pits  (cf.  Fig.  53). 

7.  Tracheae  or  vessels  (spiral)  ;  scattered  here  and  there  among  the 
tracheids,  and  hardly  distinguishable  from  them  in  cross-section.  They 
are  continuous  elongated  tubes  filled  with  air,  and  strengthened  by  a  b<'au- 
tiful  close  spiral  ridge  (sometimes  double)  which  runs  round  the  inn«T  face 
of  the  wall  (Fig.  52). 

The  tracheids  and  vessels  are  of  great  physiological  importance,  being 
probably  the  main  channels  for  the  flow  of  sap.  Sap  is  water  holding 
various  substances  in  solution.  The  water  enters  by  the  roots,  flows  prin- 
cipally through  the  walls  of  the  vessels  and  tracheids,  and  not  through 
their  cavities,  which  are  filled  with  air,  and  is  thus  conducted  through  the 
rhizome  and  upwards  into  the  leaves. 

8.  Wood-parenchyma  ;  cells  like  those  of  the  phloem-parenchyma  (5) 
scattered  between  the  vessels  and  tracheids. 


122 


THE  BIOLOGY  OF  A  PLANT. 


Branches  of  the  Rhizome  These  repeat  in  all  respects  the 
structure  of  the  main  stem.  Thej  are  equivalent  members  of 
the  underground  part,  and  differ  in  no  wise,  excepting  in  their 
origin,  from  the  main  stem  itself. 

Roots.  The  roots  may  easily  be  recognized  by  their  small 
size  and  tapering  form,  and  their  lack  of  the  lateral  ridges  of  the 


Fig.  54.  (After  De  Bary.)— Sieve-tubes  from  the  rhizome  of  Pteris  aquiJina,  show- 
ing: A,  the  end  of  a  member  of  a  sieve-tube  ;  B,  part  of  a  thin  longitudinal  sec- 
tion. The  section  has  approximately  halved  two  sieve-tubes,  S>  and  S^ ,  which  are 
so  drawn  that  the  uninjured  side  lies  behind.  The  broad  posterior  surface  of  S^ 
is  seen  covered  with  sieve-plates  connecting  with  another  sieve-tube.  S^  on  the 
contrary,  abuts  by  a  smooth  non-plated  surface  upon  parenchymatous  cells 
which  are  seen  through  it.  w,  sections  of  walls  bearing  sieve-pits ;  x,  section  of 
a  non-plated  wall  abutting  upon  parenchyma. 

stem  and  branches.  They  arise  endogenously  from  the  main 
stem  or  its  branches,  i.e.,  by  an  outgrowth  of  the  internal  tissues, 
and  not  (as  in  the  case  of  the  false  roots  or  rhizoids  of  the  pro- 
thallium,  shortly  to  be  described)  by  elongation  .of  superficial 
cells  of  the  epidermis.  True  roots,  of  which  those  of  Pteris  are 
good  examples,  arise  always  as  well  from  the  fundamental  and 
fibro-vascular  regions,  and  include  all  the  systems  found  in  the 
stem  itself.  Hence  cross- sections  of  Pteris  roots  differ  but 
slightly  from  those  of  the  stem  or  the  branches,  and  the  root  in 
general  is  clearly  a  member  of  the  plant  body.  As  in  all  true 
roots,  the  free  end  is  covered  by  a  special  boring  tip  called  the 


STRUCTURE  OF  THE  APICAL  BUDS. 


123 


root-cap.,  but  this  is  apt  to  be  lost  in  removing  the  specimen 
from  the  earth. 

The  Embryonic  Tissue  or  Meristem  of  the  Rhizome.  The 
mature  rhizome  remains  at  the  tip  nearly  undiiterentiated  into 
tissues.  At  tliis  point  the  epidermis  may  ])e  distinguished,  but 
it  remains  very  delicate,  and  thu  underlying  cells  continue  to 
grow  and  multiply,  producing  continued  elongation  of  the  mass. 
In  this  way  the  apical  bud  is  formed.  Lateral  buds  are  given 
off  right  and  left  to  constitute  the  embryos  of  leaves,  branches, 
or  roots,  wliich,  always  retaining  their  soft  and  delicate  tips,  are 
capable  of  further  growth. 

Behind  these  ''growing  points"  the  epidermis  and  otlier 
tissues  grow  more  and  more  slowly,  and  soon  reacli  their  maxi- 
mum size,  whereupon  rapid  growth  ceases.  The  power  of 
growth  is  henceforward  mainly  conlined  to  the  apical  buds,  and 
the  growing  tissue  of  which  they  are  composed  is  known  as  em- 
hryotiic  {issue  or  meristem. 

The  Apical  Cell  of  the  Rhizome.  Close  examination  reveals 
the  fact  that  each  apical  bud  contains  a  remarkable  cell  which  is 
especially  concerned  in  the  function  of  growth,  viz.,  the  apical 
cell.)  which  lies  in  a  hollow  at  the  apex  of  the  bud.  In  the 
apical  buds  of  the  rliizome  or  branches  this  cell  has  somewhat  the 


a,c. 


m. 


Fig.  55a.  (After  Hofmeister.)— Apical  cell 
of  the  rhizome  in  a  vertical  longitudinal 
section,  a.c^  apical  cell ;  /»,  hair ;  m,  meri- 
stem. 


Fig.  55n.  (After  Hofmeister.)  — 
Apical  cell  of  tlie  rliizome  in  hori- 
zontal longitudinal  section,  a.c^ 
apical  cell. 


form  of  a  wedge  with  its  base  turned  forwards  and  its  thin  edge 
backwards,  the  latter  placed  at  right  angles  to  a  plane  passing 
through  the  lateral  ridges.  It  continually  increases  in  size,  but 
as  it  grows  repeatedly  divides   so  as  to    cut  oif  cells  laterally 


124 


THE  BIOLOGY  OF  A  PLANT. 


alternately  on  its  riglit  and  left  sides.  These  cells  in  turn  con- 
tinue to  grow  and  divide,  and  thus  give  rise  to  two  similar  masses 
of  meristem,  which  together  constitute  the  apical  bud.  From 
the  meristem  by  gradual,  though  rapid,  changes  the  various  tis- 
sues of  the  adult  rhizome  are  differentiated ;  and  longitudinal 
sections  passing  through  the  lateral  ridges  show  the  mature 
tissues  fading  out  in  a  region  of  indifferent  meristem  about  the 
apical  cell  (Fig.  55b). 

The  apical  cell  lies  at  the  bottom  of  a  funnel-shaped  depression  at  the 
tip  of  the  stem.  It  is  shaped  approximately  like  a  thin,  two-edged  wedge 
w'ith  an  arched  or  curved  base  turned  forwards  towards  the  centre  of  the 
funnel-shaped  depression.  The  thin  edge  of  the  wedge  is  directed  back- 
wards, and  its  sides,  which  are  also  curved,  meet  in  a  vertical  plane  above 
and  below.     A  longitudinal  section  taken  through  the  plane  of  the  lateral 


a<o 


Fig.  56.    (After  Sachs.)— A  vertical  transverse  section  through  the  apical  cell,  a.Ct 
showing  a  boundary  of  hairs  and  a  second  apical  cell,  I,  belonging  to  a  leaf. 

ridges  therefore  shows  the  apical  cell  in  a  triangular  form  as  in  Fig.  55b. 
A  section  taken  at  right  angles  to  this — i.e.,  vertical  and  longitudinal — 
shows  the  cell  to  be  approximately  rectangular  and  quadrilateral  (Fig. 
55a),  while  a  transverse  vertical  section  shows  it  in  the  form  of  a  bi-convex 
lens  (Fig.  56). 

The  funnel-shaped  depression  is  compressed  vertically,  and  its  w^alls  are 
thickly  covered  with  erect  branching  hairs,  w^hich  are  closely  fastened 


m. 


Fig.  57.— Cross-section  of  an  entire  fertile  leaflet,    m.r,  midrib ;  v,  veins ;  ep,  epi- 
dermis ;  ms,  mesophyll ;  sp,  sporangia ;  in,  indusium. 

together  by  a  hardened  mucilage  secreted  by  the  apical  bud.     These  hairs 
entirely  close  the  mouth   of  the   funnel  and  shut  off  the  delicate  young 


HISTOLOGY  OF  TUE  LEAF. 


125 


portions  at  its  base  from  the|Outer  world.  Protected  by  these  hairs,  the 
end  of  the  stem  forces  its  wayfthrough  the  toughest  clay  without  injury  to 
the  delicate  bud  buried  in  its  apex.     (Hofmeister.) 


Fig.  58.— Cross-section,  still  more  enlarged,  passing  through  the  midrib  of  a  leaflet. 
In  the  centre  the  circular  fibro-vascular  bundle,  supported,  especially  above  and 
below,  by  thickened  prosenchyma  ( p).  On  either  side  the  parenchymatous,  mes- 
ophyll  cells  (shaded)  and  the  intercellular  spaces  (j.j.)  opening  by  stomata  (st)', 
epidermis  (ep). 


The  Aerial  Part  of  the  Brake.     The  Frond  or  Leaf. 

The  external  form  of  the  leaf  has  been  descril)ed  on  p.  109, 
and  it  now  remains  to  consider  its  internal  structure.  The 
lamina  is  to  be  regarded  as  a  flattened  and  ahered  portion  of  the 
stipe,  made  thin  and  delicate  in  order  to  present  a  large  surface 
to  the  light  and  the  air.  The  stipe,  in  turn,  is  a  prok^ngation 
of  the  rhizome,  so  that  the  whole  plant  body  is  a  continuous 
mass,  throughout  wdiicli  extend  tlie  three  systems  of  tissue  vir- 
tually unchanged.  Tlie  transverse  and  longitudinal  sections  of 
the  stipe  show  only  minor  points  of  difference  from  correspond- 
ing sections  of  the   rhizome.      In  the  leaf,  however,  all  tliree 


126 


THE  BIOLOGY  OF  A  PLANT. 


systems  undergo  great  changes.  The  epidermis  becomes  very 
thin,  dehcate,  and  transparent ;  the  fibro-vasciilar  bundles  break 
up  into  an  extremely  fine  and  complex  network  forming  the 


Fig.  59.— Cross-section  of  part  of  a  leaflet  showing  the  microscopic  structure,  ep, 
epidermis ;  st,  stomata ;  r.s,  intercellular  spaces  between  the  mesophyll-cells, 
which  are  filled  with  (shaded)  chlorophyll-bodies  lying  in  the  protoplasm. 

veins  I  the  sclerotic  tissues  become  transparent  and  are  found 
only  along  the  veins.  The  cells  of  the  fundamental  parenchyma 
alter  their  form,  lose  their  starch,  and  become  filled  with  bright- 
green,  rounded  bodies,  called  the  cliromatopliores  or  GMoropJiyll- 
hodies^  which  are  composed  of  a  protoplasmic  basis  colored  by  a 
pigment  known  as  chlorophyll.  The  green  fundamental  paren- 
chyma of  the  leaf  is  sometimes  called  the  mesophyU. 

A  cross-section  of  a  leaflet  (p.  109)  is  shown  in  Fig.  57. 
The  finer  structure  of  the  leaflet  is  shown  in  Fiffs.  ^S  and  59. 
On  the  outside  is  the  epidermis  (ep) ;  within,  the  mesophyll  and 
midrib — the  latter  composed  of  thickened  epidermal  and  sclerotic 
fundamental  tissue,  and  a  large  fibro- vascular  bundle. 

The  mesophyll^  or  leaf -parenchyma,  consists  of  irregular  cells 


HISTOLOGY  OF  THE  LEAF. 


127 


which  are  loosely  arranged  on  the  lower  side,  leaving  very  large 
intercellular  spaces,  but  are  closely  packed,  and  leave  few  or  no 
intercellular  spaces,  on  the  iip])er  (sunny)  side.  The  cells  have 
very  thin  walls,  contain  protoplasm   and  a  large  central  space 


Fig.  60.— Epidermis  from  the  under  side  of  a  leaflet,  showing?  wa\'y  cells ;  elonprated 
(pro,sencAjj/ma^)».s)  cells  over  the  veins;  and  stomata  with  tlieir  guard-cells,  sty 
stomata  and  guard-cells ;  v,  veins  covered  by  thick  and  prosenchymatous  epi- 
dermal cells.  Intermediate  stages  between  wavy  and  straight  cells  are  also 
shown.    (Surface  view.) 

(vacuole)  filled  with  sap,  and  numerous  chlorophyll-bodies  im- 
bedded in  the  protoplasm.     These  are  especially  numerous  in 


128 


THE  BIOLOGY  OF  A  PLANT. 


c. 


the  upper  part  of  the  leaf,  as  might  be  expected  from  theit 
functions  in  connection  witli  the  action  of  light  (see  page  147). 

The  epidermis^  or  shin  of  the  leaf,  consists  of  translucent, 
greatly  flattened  cells  having  peculiar  wavy  outlines  and  rela- 
tively thick  walls  (Figs.  58-61).  Upon 
the  veins  they  become  elongated,  and 
their  walls  are  considerably  thickened, 
especially  upon  the  midrib  (Fig.  58, 
They  generally  contain  large,  distinct 
nuclei,  and  often  considerable  proto- 
j^lasm.  The  wavy  epidermal  cells, 
particularly  in  young  plants,  contain 
some  chlorophyll  and  starch,  though 
1-c*  in  this  respect  the  fern  is  somewhat 
exceptional. 

In  the  rhizome  the  epidermis  forms 
a  continuous  layer  over  the  whole  sur- 

(After  Sachs.)— Epi-  facc.      In  the  leaf,  however,  this  is  not 
dermal  cells  of  ptcris  flabei-  ^]^g  ^^^^    ^l^g  epidermis  ou  the  lower 

tola,  showing  the  development      ^  ^  ^  ^ 

'•  of  stomata.    A,  very  young  side  being  perforated  by  holes  leading 
Z?::-rL.!lima.t,ric^  ^^to  the  interior  and  known  as  months 

mother-cell;     sx,    sudsidiary     or   stOlJiata  (singular,   StomO)  (Fig.    61). 
cell ;  g.c,  guard-cell ;  St,  stoma,    ^m  ii  i  x  -xxi  n 

Ihese  holes  do  not  pass  into  the  ceils, 
but  are  gaps  or  breaks  between  certain  cells  of  the  epidermis, 
and  open  directly  into  the  intercellular  spaces,  of  wliich  they  are, 
in  fact,  the  ends.  That  portion  of  the  intercellular  labyrinth 
which  directly  underlies  the  stoma  is  sometimes  called  the  respira- 
tory cavity.  Each  stoma  is  l)ounded,  as  in  most  plants,  by  two 
curving  guard-cells^  which  are  generally  nucleated,  and,  unlike 
epidermal  cells  generally,  contain  abundant  chlorophyll-bodies 
and  starch. 

The  guard-cells  are  capable  of  clianging  their  form  accord- 
ing to  the  amount  of  light,  the  hygroscopic  state  of  the  atmos- 
phere, and  other  circumstances,  and  thus  open  or  close  the  hole 
or  stoma  between  them.  This  action  is  of  great  importance  in 
the  physiology  of  the  plant  (transpiration,  p.  117). 

In  Pteris  cretica  and  P.  fldbellata  the  stomata  develop  as  follows  :  A 
young  epidermal  cell  is  divided  by  a  curved  partition  into  two  cells,  one  of 
which  (Fig.  61)  is  called  the  initial  cell  of  the  stoma  {i.c).     This  is  again 


VENATION.  129 

divided  by  a  curved  partition  into  the  mother-cell  of  the  stoma  (Fig.  61, 
m.c)  and  a  subsidiary  cell  (Fig.  61,  s.c). 

The  mother-cell  is  then  bisected  into  the  two  guard-cells,  and  the  stoma 
appears  as  a  chink  between  them  (Fig.  61,  B). 

The  veins  are  the  fibres  or  threads  which  constitute  the 
framework  of  the  leaf.  Each  consists,  essentially,  of  a  small 
iibro-vascular  bundle  branching  from  that  of  the  midrib  (Figs. 
57,  58,  62).  Above  and  below  them  the  mesophyll  and  epi- 
dermal cells  are  generally  thickened  and  prosenchymatous,  in  this 
way  contributing  alike  to  the  form  and  the  function  of  the 
*'  vein." 


Fig.  63.    (After  Luerssen.)— Venation  of  a  leaflet  of  Pteris  aquilina. 

Their  arrangement  (veining  or  venation)  is  definite,  and  depends  on  the 
mode  of  branching  of  the  fibro-vascular  strand  which  constitutes  the  prin- 
cipal part  of  the  midrib.  Secondary  strands  (nerves)  proceed  from  this  at 
an  acute  angle,  then  turn  somewhat  abruptly  towards  the  edge  of  the 
leaflet  (or  lobe),  making  an  arch  which  is  convex  towards  the  distal  ex- 
tremity of  the  midrib  (Fig.  62). 

From  this  point,  after  branching  once  or  twice,  the  delicate  veins  run 
parallel  to  each  other  to  the  edge  of  the  leaflet,  where  they  join  one  another 
or  anastomose.  This  form  of  venation  is  known  as  Nervatio  Neuropteri- 
dis,  and  is  more  easily  seen  in  the  leaf  of  Osmunda  regalis  (cf.  Luerssen, 
Rabe7ihorsVs  Kryptogamen-Flora  (1884),  III.,  s.  12). 


CHAPTEE  IX. 

THE  BIOLOGY  OF  A  PLANT  {Continued). 

Reproduction  and  Development  of  the  Brake  or  Fern, 

Reproduction.  Unlike  tlie  earthworm,  the  fern  reproduces 
both  by  gamogenesis  (sexually)  and  agamogefiesis  (asexually). 
Pteris  possesses  two  modes  of  asexual  reproduction,  viz.,  the 
detachment  of  entire  branches  from  the  rhizome  and  the  con- 
sequent establishment  of  independent  plants,  as  already  men- 
tioned (p.  Ill),  and  the  formation  of  "  adventitious  buds  "  from 
the  bases  of  the  leaf- stalks  (Fig.  46).  But  besides  these  the 
fern  has  a  quite  different  method  of  reproduction,  in  which  a 
process  of  agamogenesis  regularly  alternates  with  gamogenesis 
{alternation  of  generations).  The  folio  whig  brief  outhne  of 
this  important  process  may  help  to  guide  the  student  through 
the  subsequent  detailed  descriptions. 

Upon  some  of  the  leaves  are  formed  organs  called  sijorangia 
(Figs.  57,  63,  64),  which  produce  numerous  reproductive  cells 
called  spores.  The  spores  become  detached  from  the  parent  and 
develop  into  indejDendent  plants,  the  prothallia  (Fig.  70),  w^hich 
differ  entirely  in  appearance  from  the  fern  and  ultimately  pro- 
duce male  and  female  germ-cells.  The  female  cell  of  the  pro- 
thallium,  if  fertilized  by  a  male  cell,  develops  into  an  ordinary 
' '  fern, ' '  which  again  produces  spores  asexually.  The  forma- 
tion and  development  of  the  spores  is  evidently  a  process  of 
agamogenesis.,  and  the  fern  proper  is  therefore  neither  male  nor 
female — i.e.,  it  is  sexless  or  asexual.  The  formation  and  de- 
velopment of  the  germ-cells,  on  the  contrary,  is  a  process  of 
gamogenesis  j  and  the  prothallium  is  a  distinct  sexual  plant, 
being  both  male  and  female  (Jiermajyhi'odite  or  hisexual).  In 
general  terms  this  is  expressed  by  calling  the  ordinary  fern  the 
spore-bearer,  or  sporophore^  and  the  prothallium  the  egg- 
bearer,    or    oopJiore.       The   life -history    of   the    fern,    broadly 

130 


ALTERNATION  OF  GENERATIONS. 


131 


Fig.  63.  (After  Suminski.)— Spo- 
rangium of  Pteris  serrulata.  p, 
pedicel ;  c,  capsule ;  a,  annulus ; 
s,  spore. 


speaking,  consists  tlierefore  in  an  alternation  of  the  sporophore 
(asexual  generation)  with  the  oopliore  (sexual  generation) ;  that 
is,  it  consists  of  an  alternation  of 
generations.  An  essentially  similar 
alternation  of  sporophore  with  oophore 
occurs  in  all  higher  plants,  though  in 
most  cases  it  is  so  disguised  as  to  es- 
cape ordinary  observation. 

The  Sporangia  and  Spores.  The 
sporangia  of  Pteris  (Figs.  63,  64) 
arise  upon  a  longitudinal  thickening 
of  tissue  situated  on  the  under  side  of 
the  leaflets  near  their  edges,  and  in- 
cluding a  marginal  anastomosis  of  the 
veins.  This  swelling  is  known  as 
the  receptacle.  Hairs  are  not  uncom- 
mon upon  the  under  side  of  the  leaf, 
and  some  are  found  upon  or  near  the 
receptacle.  On  the  latter  arise  structures,  at  first  superficially 
similar  to  hairs,  which  become  enlarged  at  the  tip,  and  finally 
develop  into  the  sporangia.  Meanwhile  the  edge  of  the  leaflet 
is  bent  down  and  under  so  as  to  make  a  longitudinal  band  of 
thin  tissue  composed  of  epidermis  known  as  the  oider  veil  or 
indusium  (Fig.  64,  o.i).  A  similar  thin  sheet  of  epidermis 
grows  down  from  the  under  side  of  the  leaf,  and  passing  out- 
wards to  meet  the  former,  constitutes  the  inner  veil  or  true 
indusium  (Fig.  64,  B^  i.i). 

In  the  Y-shaped  space  thus  formed  the  sporangia  are  de- 
veloped. 

A  superficial  (epidermal)  cell  enlarges  and  becomes  divided  into  a 
proximal  (basal)  cell  and  a  distal  (apical)  cell  (Fig.  65,  a).  The  former  de- 
velops into  the  iwiuve  pedicel  or  stalk  of  the  sporangium  ;  the  latter  gives 
rise  to  the  head  or  capsule  within  which  the  spores  are  formed  (cf.  Fig.  63>. 
The  pedicel  arises  from  the  original  pedicel-cell  by  continued  growth  and 
subdivision  until  it  consists  of  three  rows  of  cells  somewhat  elongated. 
The  rounded  capsule-cell  is  next  transformed  by  four  successive  oblique 
divisions  into  four  plano-convex  "parietal  cells"  and  a  tetrahedral  central 
cel>,  the  archesporium,  enclosed  by  the  others.  The  capsule-cell  is  thus 
divided  by  three  planes  inclined  at  about  120°  (Fig.  65,  6,  c).  A  fourth 
(Fig.  65,  d,  e)  passes  nearly  parallel  to  the  top  of  the  capsule  and  cuts  off 


132 


THE  BIOLOGY  OF  A  PLANT. 


from  it  the  central  cell  or  archesporiiim.  In  the  parietal  cells  further 
divisions  follow,  perpendicular  to  the  surface,  while  the  archesporium  gives 
rise  to  four  intermediate  or  tapetal  cells,  parallel  to  the  original  parietal 
group  (Fig.  65,  g).  The  sporangium  now  consists  of  a  central  tetrahedral 
archesporium  bounded  by  four  tapetal  cells,  which  in  turn  are  enclosed  by 
the  parietal  cells,  at  this  time  rapidly  multiplying  by  divisions  perpen- 
dicular to  the  exterior.     Owing  to  the  peculiar  position  of  the  planes  of 


B. 


Fig.  64.  (From  Luerssen,  after  Burck.)— Indusiaand  receptacle  of  Pterisaquilina; 
B  (diagrammatic),  seen  from  below ;  A,  in  the  section  of  the  edge  of  a  leaflet,  o.i, 
outer  (false)  indusium ;  i.i^  inner  (true)  indusium ;  r,  receptacle ;  s,  young 
sporangia. 

\ 

division  the  whole  capsule  is  now  somewhat  flattened,  and  it  becomes  still 
more  so  by  the  formation  along  the  edge  of  a  peculiar  structure  called  the 
ring  or  annulus^  whose  function  is  the  rupturing  of  the  capsule  and  the 
liberation  of  the  spores.  The  annulus  is  formed  by  a  number  of  parallel 
transverse  partitions  (Fig.  65,/,  h,  i,J),  which  subdivide  the  peripheral 
cells  of  one  edge  of  the  capsule  until  a  certain  number  of  cells  have  been 
formed.  These  then  project  upon  the  capsule  (Fig.  65,  J)  and  form  an  in 
complete  ring  (Fig.  65,  k). 

Meanwhile  the  tapetal  cells  sometimes  subdivide  so  as  to  form  a  double 
row  (Fig.  65,  h),  and  soon  afterwards  are  absorbed,  space  being  thus  left 


DEVELOPMENT  OF  SPORANGIA. 


133 


Fig.  65.  (After  Luerssen.)— Development  of  the  sporangia  of  Aspklmm  Filix  mas, 
which  is  closely  similar  to  that  of  Pterin,  a,  the  young  sporangium  standing 
upon  the  epidermis-cell  from  which  it  has  just  been  divided;  r,  the  proximal 
cell  cut  off  from  the  sporangium  to  form  the  pedicel  and  support  the  capsule; 
a,  1,  the  first  partition  in  the  capsule;  b,  1  and  2,  the  first  and  second  partitions; 
c,  1,  2,  4,  the  first,  second,  and  fourth  partitions ;  d  and  e  are  cross-sections  of  the 
capsule  showing  the  oV^lique  position  of  the  partitions,  and  especially  that  of  the 
third  ;  /,  a  later  stage  ;  3,  the  origin  of  the  tapetal  cells  and  the  formation  of  tlie 
archesporium ;  /«,  division  of  the  tapetal  cells  and  the  formation  of  the  spore 
mother-cells;  I,  four  spores  as  they  originate  in  the  spore  mother-cells;  i,  j,  fc, 
the  annulus  and  ripe  sporangium,  in  surface  view ;  j),  peripheral  cells;  ar, 
archesporium  ;  t,  tapetal  cells ;  an,  annulus. 


134 


THE  BIOLOGY  OF  A  PLANT. 


for  the  growth  and  enlargement  of  the  archesporium.  The  latter  now 
divides — first  into  2,  then  into  4,  8,  and  finally  16  cells,  the  mother-cells 
of  the  S2)ores.  These  remain  for  a  time  closely  united,  but  eventually 
separate  and  again  subdivide,  each  into  4  daughter-cells  (Fig.  65,  I).  The 
64  cells  thus  formed  are  the  asexual  sjjoi^es.  In  their  mature  state  they 
have  a  tetrahedral  form  and  certain  external  markings,  indicated  in  Figs. 
63,  66.  Each  spore  acquires  a  double  membrane,  viz.,  an  inner,  endo- 
sporium,  delicate  and  white,  and  an  outer,  exosjjormm,  yellowish  brown, 
hard,  and  sculptured  over  the  surface  with  very  close  and  fine,  but 
irregular,  warty  excrescences. 

Germination  of  the  Spores.  Development  of  the  Prothallium. 
In  the  brake  the  spores  ripen  in  July  or  August  and  are  set 
free  by  rupture  of  the  sporangium  under 
the  strain  exerted  by  the  elastic  annulus,  as 
indicated  in  Fig.  63.  Germination  of  the 
spores  normally  occurs  only  after  a  considera- 
ble period  (perhaps  not  before  the  following 
spring) ;  it  begins  by  a  rupture  of  the  exospo- 


Fio.  66.  (After 
Suminski.)  — 
Single  spore  of 
Pteris  scrrula- 
ta. 


Fig.  67.  (After  Suminski.)— Germinat- 
ing spores  of  Pteris  serrulata.  A,  in  an 
early  stage  ;  B,  after  the  appearance 
of  one  transverse  partition ;  s,  spore  ; 
p,  protonema ;  r,  rliizoid. 


Fig.  68.  ^After  Sumin- 
ski.)—Very  young  pro- 
thallium  of  Pteris, 
showing  the  spore  (s), 
two  rhizoids  (r),  and 
the  enlarging  extrem- 
ity. 


rium  which  is  probably  immediately  due  to  an  unbibition  of 
water.  The  spore  bursts  irregularly  along  the  borders  of  the 
pyramidal  surfaces,  and  from  the  opening  thus  formed  the  endo- 
sporium  protrudes  as  a  papilla  filled  with  protoplasm  in  which 
numerous  chlorophyll-bodies  soon  aj^pear. 

This  papilla  is  known  as  the  ijrotonema.^  or  first  portion  of 
the  prothallium  (Fig.  67).  It  develops  very  quickly  into  a  stout 
cylindrical  protrusion  divided  into  cells  joined  end  to  end. 
Close  to  the  spore  one  or  more  rhizoids  are  put  down  from  the 


DEVELOPMENT  OF  THE  PliOTHALLIUM. 


135 


growing  protonenia  to  serve  as  anchors  and  roots.  At  the  oppo- 
site or  distal  end  longitudinal  partitions  soon  appear  (Fig.  68), 
whieli  speedily  convert  this  portion  into  a  broad  flat  plate  at 
first  only  one  cell  thick,  but  eventually  several  cells  tliick  al(jng 
the  median  line.  This  thickening  is  the  so-called  "cushion" 
(see  Fig.  70).  The  whole  prothalliuni  is  now  somewhat  spatulate 
(Fig.  69),  but  by  further  grow^th  anteriorly,  by  an  apical  cell  or 
otherwise,  the  wider  end  becomes 
still  more  ilattened  and  heart- 
shaped  or  even  kidney -shaped. 
Numerous  rhizoids  (so-called  be- 
cause they  are  not  morphologi- 
cally true  roots)  are  put  down, 
and  the  whole  structure  assumes 
approximately  the  appearance  in- 
dicated in  Fig.  70.  The  spore- 
membranes  and  proton  ema  soon 
fall  away,  and  the  prothallium 
enters  upon  an  independent  exist- 
ence, being  rooted  by  its  rhizoids 
and  having  an  abundance  of 
chlorophyll.  In  the  broad  thin 
plate  of  tissue  no  subdivision  into 
stem  and  leaf  exists,  and  the 
plant  body  closely  resembles  the 
' '  thallus ' '  of  one  of  the  lowest 
plants.  Since  it  is  the  precursor 
of  the  ordinary  "fern,"  it  is 
called  the  '' prothallus  "  or  '''pro- 
thallium. ' ' 

The  cushion  forms  a  prominence  on  the  lower  side ;  upon 
its  posterior  part  most  of  the  rhizoids  are  borne. 

Sexual  Organs  of  the  Prothallium.  The  prothallia  of  ferns 
are  as  a  rule  bisexual  or  hermaphrodite ;  that  is,  each  individual 
possesses  both  male  and  female  organs.  But  the  latter  appear 
somewhat  later  than  the  former,  and  poorly  nourished  prothallia 
often  bear  only  male  organs,  though  they  will  frequently  develop 
female  organs  also  if  placed  in  better  circumstances. 

The  Antheridia^  or  male  organs,  are  hemispherical  promi- 


FiG.  69,  (After  Suminski.)— Older  pro- 
thallium, showing  two  rhizoids,  three 
young  antheridia,  and  numerous 
chlorophyll-bodies. 


136 


THE  BIO  LOOT  OF  A  PLANT. 


nences  occurring  upon  the  posterior  part  and  the  under  side  of 
the  prothallium,  often  among  the  rhizoids.  When  fully  formed 
(Fio-s.  70,  71)  an  antheridium  consists  of  a  mass  of  rounded  cells 
{sperinatozoid  mother-cells)  enveloped  by  a  membrane  one  cell 
In  tliickness. 


i.h-. 


m 


Fig.  70.  (After  Suminski,  slightly  modified.)— Adult  prothallium  of  Fter\B  serrulata 
seen  from  below,  showing  the  rhizoids  (r)  at  the  posterior  end,  the  depression  at 
the  anterior  end ;  the  cushion  near  the  latter  bearing  (in  this  case)  four  arche- 
gonia.  Among  the  rhizoids  are  the  (spherical)  antheridia.  The  chlorophyll- 
bodies  only  are  shown  in  the  cells  of  the  broad  plate  of  tissue  constituting  the 
prothallium.  Just  above  the  anterior  depression  is  seen  a  rrothallium  of  the 
natural  size. 


Fig.  71.  (After  Strasburger.)— Mature  an- 
theridium of  Pterin- sej7tt?<xf  ft.  p,  periphe- 
ral cells;  m,  mother-cells  of  the  sper- 
matozoids. 


Fig.  72.— Diagram  to  illustrate  the  ori- 
gin of  an  antheridium.  A,  very 
young  stage;  B,  older;  a,  original 
epidermal  cell  enlarged  ;  b,  mother- 
cell  of  the  entire  antheridium. 


MALE  GERM-CELLS. 


137 


The  mode  of  origin  of  the  mother-cells  differs  considerably  in  different 
ferns,  but  in  all  cases  is  essentially  as  follows :  An  ordinary  cell  on  the 
lower  side  of  the  prothallium  swells  and  forms  a  hemispherical  or  dome- 
shaped  projection,  which  is  soon  separated  by  a  partition  from  the  original 
cell  (Fig.  72).  Further  divisions  then  follow  in  the  dome-shaped  cell  such 
that  a  central  cell  is  left,  surrounded  by  a 
layer  of  peripheral  cells  (Fig.  73).  By  re- 
peated divisions  the  central  cell  splits  up 
into  the  spermatozoid  mother-cells  (Fig.  71). 

Within  each  mother-cell  the  proto- 
plasm arranges  itself  in  a  peculiar 
spiral  body,  the  sjyermatozoid^  which 
is  the  male  germ^cell. 

When  the  mature  antheridium  is 
moistened,  the  peripheral  cells  swell 
and  thus  press  out  the  mother-cells 
and  spermatozoids  (Fig.  74).  The 
latter  escape  from  the  mother-cells  and  swim  about  very  actively 
in  the  water.  They  appear  as  naked  single  cells,  of  a  peculiar 
corkscrew  shape,  and  bear  upon  the  finer  spirals  numerous  ex- 
tremely active  cilia  (p.  31),  by 
which  they  are  driven  swiftly 
through  the  water. 

The   Archegonia^    or    female 


Fio.  73.  (After  Hofmeister.)  — 
Later  stage  in  the  development 
of  an  antheridium  of  Pterix  ser- 
riilata.  p,  peripheral  cell;  c, 
central  cell  from  which  the 
spermatozoid  mother  -  cells 
arise. 


Fig.  74.  (After  Luerssen.)— Bursting  of 
the  antheridium  and  escape  of  the 
spermatozoids.  a»,  antheridium;  m.e, 
spermatozoid  mother-cells;  sp,  sper- 
matozoids. 


Fig.  75.  (After  Strasburger.)— Mature 
archegonium,  showing  the  oosphero 
(o),  the  neck  (x),  and  mucus  (»»)  is- 
suing from  the  mouth  of  the  canal. 


138 


THE  BIOLOGY  OF  A  PLANT. 


organs  (Figs.  TO,  75),  described  for  the  first  time  by  Siiminski 
in  1864,  likewise  arise  from  single  superficial  cells  of  the  pro- 
thallium.  They  are  situated  almost  exclusively  upon  the  cushion 
near  its  anterior  or  apical  extremity,  and  hence  at  the  bottom  of 
the  anterior  depression  (Fig.  70).  Since  they  appear  later  than 
the  antheridia,  they  are  not  likely  to  be  fertilized  by  spermato- 
zoids  descended  from  the  same  spore.  This  phenomenon  of 
maturation  of  one  set  of  sexual  organs  of  a  bisexual  individual 
before  the  rij^ening  of  the  other  set  is  a  common  feature  among 
plants,  and  is  known  as  dichogamy.  Tliere  is  reason  to  believe 
that  important  advantages  are  gained  by  thus  securing  cross-fer- 
tilization and  preventing  self-fertilization  or  ' '  breeding  in  and 


m 


55 


In  the  development  of  the  archegonium  the  original  cell  enlarges,  be- 
comes somewhat  dome-shaped,  and  divides  by  transverse  partitions  into 
three  cells  :  a  proximal,  im- 
bedded in  the  tissue  of  the 
prothallium,  a  middle,  and  a 
distal  dome-shaped  cell  (Fig. 
76).  The  fate  of  the  proximal 
cell  is  unimportant.  The  dis- 
tal cell  gives  rise  by  division 
to  a  chimney-like  structure, 
the  neck  (Figs.  75,  77),  which 


B. 


A, 


Fig.  76.— Diagram  to  iUustrate 
the  origin  ot  an  archegonium. 
A^  an  early  stage;  B,  a  later 
stage;  A,  a,  the  original  epi- 
dermal cell  enlarged ;  B,  a,  the 
basal  cell;  b,  the  central  or 
canal  cell ;  c,  the  neck-cell. 


Fig.  77.  (After  Strasburger.)— Developing  arche- 
gonia  of  Pteris  serrulata.  A,  young  stage ;  J3, 
older ;  n,  neck ;  c,  canal ;  o,  oosphere. 


encloses  a  row  of  cells  (canal-cells)  derived  from  the  original  middle  cell 
(Figs.  75,  77).  These  afterwards  become  transformed  into  a  mucilaginous 
substance  filling  a  canal  leading  through  the  neck  from  the  outside  to  the 
oosphere  (Fig.  77),  which  also  arises  from  the  original  "middle"  cell  at  its 


FERTILIZATION  AND  DEVELOPMENT.  139 

proximal  end.    Theoospbere  is  the  all-important /e/TiaZe  germ-cell  to  which 
the  "  neck-"  and  "canal-cells"  are  merely  accessory. 


.^7^. 


Fertilization  or  Impregnation.  Fertilization,  or  the  sexual 
act,  is  perfonned  as  follows  :  Sper- 
matozoids  in  vast  numbers  are  at- 
tracted to  the  mouths  of  the  arche- 
gonia  and  there  become  entangled 
in  the  nmcilage  (Fig.  78).  In 
favorable  cases  one  or  more  work 
their  w^ay  down  the  mucilaginous 
canal,  and  at  length  one  penetrates 
and  fuses  with  the  oosphere. 

It  is  known  that  one  spermatozoid  is 

enough    to   fertilize  the    oosphere,    and   ^        r,o     ,  k  ^.        c.     -l. 

^  ^  '  Fig.    78.     (After    Strasburger.)  — 

probably  one  only  penetrates  it ;  but  sev-      Mouth  of  an  archegonium  of  Rc- 

eral  are  often  seen  in  the  mucilaginous      ri<i  sermiata,  crowded  with  sper- 

canal.    It  has  been  shown  that  the  muci-      i^^^tozoids  striving  to  effect  an  en- 

lage  contains  a  small  amount  (about  0.3^) 

of  malic  acid,  which  probably  acts  both  as  an  attraction  to  the  spermato- 
zoids  and  as  a  stimulus  to  their  movements.  Pfeffer  has  proved  that 
capillary  tubes  containing  a  trace  of  a  malate  in  solution  are  as  attractive 
to  the  spermatozoids  as  is  the  mucilage  in  the  central  canal,  and  phe- 
nomena of  this  kind  (chemiotaxts)  have  recently  been  shown  to  be  common 
and  highly  important. 

The  entrance  of  the  spermatozoid  into  the  ovum  and  its 
fusion  with  it  mark  an  important  epoch  in  the  life-history  of  the 
fern.  The  oosphere  is  from  this  instant  a  new  and  very  difter- 
ent  thing,  viz. ,  an  e?nhri/o,  and  is  known  as  the  oospore.  It  is 
now  the  first  stage  of  the  asexual  generation,  though  it  is  still 
maintained  for  some  time  at  the  expense  of  the  sexual  generation 
or  odpliore  (p.  130). 

Growth  of  the  Embryo.  The  oospore,  or  one-celled  embryonic 
sporophore  (p.  130),  now  rapidly  becomes  multicelhihir  by  di- 
viding first  into  hemispheres,  then  into  quadrants,  etc.  (Fig.  SO ; 
compare  Fig.  14).  The  first  plane  of  division  is  approximately 
a  prolongation  of  the  long  axis  of  the  arcliegonium  (Fig.  S(")). 
The  second  is  nearly  at  riglit  angles  to  it,  so  tliat  the  quadrants 
may  be  descril)ed  as  anterior  and  posterior  to  the  first  ph\no. 
The  fate  of  the  quadrant-cells  is  of  special  importance.     The 


140 


THE  BIOLOGY  OF  A  PLANT. 


lower  anterior  quadrant  as  it  undergoes  further  division  grows 
out  into  i\\Q  first  root  j  the  upper  anterior  quadrant  in  like  man- 
ner gives  rise  to  the  rhizome  and  \h.Q  fii'st  leaf.  The  mass  of 
cells  derived  from  the  two  posterior  quadrants  remains  connected 
with  the  prothallium  as  an  organ  for  the  absorption  of  nutri- 
ment from  the  latter,  and  is  inappropriately  called  iYiQfoot. 


ewu 


A.  ' 


Fig.  79.  Fig.  80. 

Fig.  79.  (After  Hofmeister.)— Development  of  the  embryo.  A^  section  showing  the 
closed  neck  (n)  and  the  planes  of  quadrant  division  of  the  oospore  or  embryo  {cm). 
The  fore  end  of  the  prothallium  is  to  the  right.  B  and  C,  stages  of  the  embryo 
later  than  A^  showing  the  beginnings  of  apical  growth ;  /,  foot ;  Z,  leaf ;  r,  root ; 
r?j,  rhizome. 

Fig.  80.  (From  Luerssen,  after  Kienitz-Gerloff.)— Development  of  the  embryo  of 
Pteris  serrulata.  The  figures  are  optical  sections  taken  vertically  in  the  antero- 
posterior axis  of  the  prothallium,  passing  through  the  long  axis  of  the  neck  of 
the  archegonium  :  except  C  and  D,  which  are  taken  at  right  angles  to  the  others. 
A,  a,  and  p  are  the  anterior  and  posterior  segments  of  the  oospore  after  this  has 
divided  into  hemispheres.  The  former  (a)  forms  the  stem,  the  latter  (p)  the  root. 
F  shows  in  a  late  stage  the  division  of  the  quadrants,  r  going  to  form  the  root,  s 
the  stem  or  rhizome,  I  the  leaf,  and  /  the  foot ;  1\  l,  and  s  soon  take  on  apical 
growth  as  indicated  in  H  and  I. 

In  Pteris  serrulata  the  development  is  slightly  different.  The  lower 
anterior  cell  becomes  the  first  leaf  ;  the  upper  anterior  becomes  the  first 
portion  of  the  rhizome,  the  lower  posterior  becomes  the  primary  root,  and 
the  upper  posterior  remains  as  the  '■'■foot.'''' 

The  several  parts  now  enter  upon  rapid  growth  accompanied 
by  continued  cell -multiplication,  until  a  stage  is  reached  repre- 


GROWTH  AND  DIFFERENTIATION. 


141 


sented  in  (7,  Fig.  79.  A  stage  somewhat  later  than  this,  ^vith 
its  attacliment  to  the  prothallium,  is  sliown  in  Fig.  81.  After 
tliis  the  leaf  grows  upwards  into  the  air,  the  root  downwards 
into  the  earth,  and  the  young  fern  begins  to  shift  for  itself. 
Eventually  it  reaches  a  condition  shown  in  Figs.  82  and  83. 
The  prothalHuni  remains  connected 
wdth  the  young  fern  for  some  time, 
and  may  readily  be  found  in  this 
condition  attached  to  ilower-j)ots  in 
hot-houses,  etc.  But  sooner  or 
later  it  falls  off,  and  the  young  fern 
enters  upon  an  entirely  independent 
existence.  The  appearance  of  the 
plant  and  the  shape  of  the  leaf  do 
not  always  at  first  resemble  those 
of  the  adult  fern;  growth  is  also 
more  rapid  at  first,  several  leaves 
(7-12)  being  developed  successively  in  the  first  year  (p.  112). 
Differentiation  of  the  Tissues.  In  the  earliest  stashes  the  tissue 
is  nearly  or  quite  homogeneous,  i.e.,  meristemic.  But  very 
early  in  development,  as  tlie  leaf  turns  upwards  and  the  root 


tt 


Fig.  81.  (After  Hofmeister.)— Young 
embryo  of  Pteris  aquiU)ia,  showing 
its  attachment  to  the  prothallium 
by  the  foot ;  I,  leaf  ;  /,  foot ;  i\  flrsi 
root. 


Fig.  83.  (After  Sachs.)— Older  embryo  of  maidenhair-fern  (AiJiautum)  attached  to 
the  prothallium.  Seen  in  section.  Z,  leaf;  r,  first  root;  rh,  beginning  of  the 
rhizome ;  p,  prothallium ;  rz,  rhizoids ;  ar,  archegonia. 

downwards,  changes  take  place,  which  lead  directly  to  a  differ- 
entiation into  tlie  three  great  systems  of  tissue — epidermal,  fibro- 
vascular,  and  fundamental.  Tlie  epidermal  and  fundamental 
systems  take  on  almost  at  once  the  pecuharities  which  have  al- 


142 


THE  BIOLOGY  OF  A   PLANT. 


ready  been  noted  in  the  adult,  p.  IIT.  The  fibro-vascular  system 
of  tissues  is  differentiated  a  little  later.  Different  as  the  tissues 
of  the  three  systems  are,  it  is  plain  from  their  mode  of  origin 
that  all  are  fundamentally  of  the  same  nature  because  of  their 
descent  from  the  same  ancestral  cell;  hence  every  cell  in  the 
plant  partakes  more  or  less  completely  of  the  nature  of  every 
other  cell.     The  resemblances  are  primary  and  fundamental,  the 

differences  secondary  and  derived. 
And  what  is  true  of  the  fern  in  this 
respect  is  equally  true  of  all  other 
many-celled  organisms. 

Course  of  the  Fibro-vascular  Bundles. 

Certain  features  of  the  disposition  and 
course  of  the  fibro-vascular  bundles  in  the 
embryo  and  in  the  adult  may  conveniently 
be  studied  at  this  point.  From  the  point 
of  junction  of  the  bundles  of  the  first  leaf 
and  first  root  (Figs.  79,  81,  82)  is  developed 
one  central  bundle  traversing  the  young 
rhizome  and  sending  branches  into  the  new 
leaves  and  roots  until  7-9  leaves  have  been 
formed.  After  this  time  the  rhizome 
forks,  and  the  course  of  the  fibro-vascular 
bundles  in  each  fork  is  henceforwards  com- 
Ftg.  83.  (After  Sachs.)— Young  pQ^^d.  A  lateral  depression  appears  in 
maidenhair-fern  (Adiantum)  at-  \  ^ 

tachedtotheprothallinm,p.   ?,  the  central  bundle   of  each   stem,  rapidly 

leaf;  1,  2,  the  first  and  second  increases  in  depth,    and  soon   divides  the 
^^^^^-  bundle  into  two,  one  upper  and  one  lower, 

which  are  best  recognized  in  old  specimens  (Fig.  48).  When  the  forked 
shoots  have  reached  a  length  of  about  three  inches,  these  bundles  send  out 
at  a  small  angle  towards  the  periphery  thinner,  forked  branches  which 
soon  unite  again  to  form  a  network  near  the  epidermis.  The  uppermost 
of  these  branches,  which  passes  in  the  median  line  above  the  axile  bundles, 
is  usually  somewhat  more  fully  developed,  and  almost  as  broad  as  the  lat- 
ter. This  structure  is  generally  retained  in  the  mature  rhizome  (Fig. 
48,  x).  The  number  of  peripheral  bundles  may  be  as  great  as  twelve  in  the 
cross-section.  They  anastomose  in  the  vicinity  of  the  place  of  insertion  of 
each  frond,  and  thus  form  a  hollow,  cylindrical  network,  having  elongated 
meshes  ;  but  no  connecting  branches  between  them  and  the  two  axile 
bundles  are  found  anywhere  in  the  rhizome.  The  latter  follow  an  en- 
tirely isolated  course  within   the  creeping  stem  ;  *  branches  from  them 


*  See,  however,   De  Bary,  Comp.  Anat.  Phanerogams  and  Ferns,  p.  295. 
Oxford,  1884. 


EXCEPTIONAL  MODES  OF  DEVELOPMENT.  143 

enter  the  leaves,  and  it  is  only  inside  the  leaf -stalk  that  these  ramifications 
are  met  by  branches  from  the  peripheral  network.  The  bundles  of  the 
roots  arise  only  from  the  peripheral  bundles,  but  those  of  leaves,  as  already 
said,  receive  branches  from  both  axillary  and  peripheral  bundles.  Two 
thick  brown  plates  {sclerotic  prosenchyma)  lie  between  the  inner  and 
outer  systems  of  bundles,  and  are  only  separated  from  one  another  at  the 
sides  by  a  narrow  band  of  parenchyma.  They  are  often  joined  on  one  side 
or  even  on  both,  in  the  latter  case  forming  a  tube  which  separates  the 
two  systems  of  bundles.     (Hofmeister.) 

Apogamy.  Apospory.  In  rare  cases,  e.g.,  in  Pteris  cretica^  the  ordi- 
nary alternation  of  generations  in  the  life-cycle  of  ferns  is  abbreviated  by 
the  omission  of  the  sexual  process,  and  the  immediate  vegetative  outgrowth 
of  the  sporophore  from  the  prothallium  (apogamy).  In  other  cases  there 
is  an  omission  of  the  spore  stage,  and  immediate  vegetative  development 
of  the  oophore  from  the  frond  {apospory).  (cf.  Farlow,  Quart.  Journ. 
Mic.  Science,  1874;  De  Bary,  Botan.  Zeitimg,  1878;  Druery,  etc.,  Journ, 
Royal  Mic.  Soc,  1885,  pp.  99  and  491.) 


CHAPTEK  X. 

THE  BIOLOGY  OF  A  PLANT  {CoJitinued), 
The  Physiology  of  the  Fern. 

The  brake,  like  the  eartliworm,  is  a  limited  portion  of  organ- 
ized matter  occupying  a  definite  position  in  space  and  time.  It 
is  bounded  on  all  sides  by  material  j^articles,  some  of  wliich  may 
be  living,  but  most  of  which  are  lifeless.  The  aerial  portion  is 
immersed  in  and  pressed  upon  by  an  invisible  fluid,  the  atmos- 
phere, while  the  underground  portion  is  sunk  in  a  denser 
medium,  the  earth,  which  likewise  acts  upon  it.  At  the  same 
time  the  fern  reacts  upon  the  air  and  the  earth,  maintaining 
during  its  life  an  equilibrium  which  is  disturbed  and  finally  gives 
way  as  the  life  of  the  plant  draws  to  a  close. 

The  Fern  and  its  Environment.  Those  portions  of  space, 
earth,  and  air  which  are  nearest  to  the  brake  constitute  its  imme- 
diate environment.  But  in  a  wider  and  truer  sense  the  environ- 
ment includes  the  whole  universe  outside  the  plant.  To  perceive 
the  truth  of  this  it  is  only  necessary  to  obseiwe  how  profoundly 
and  directly  the  plant  is  affected  by  rays  of  liglit  which  travel  to 
it  from  the  sun  over  a  distance  of  many  millions  of  miles,  or 
how  extremely  sensitive  it  is  to  the  alternations  of  day  and  night 
or  of  summer  and  winter.  The  plant  is  fitted  to  make  certain 
exchanges  with  its  environment,  drawing  from  it  certain  forms 
of  matter  and  energy,  and  returning  to  it  matter  and  energy  in 
other  forms.  Its  whole  life  is  an  unconscious  struggle  to  wrest 
from  the  environment  the  means  of  subsistence ;  death  and  decay 
mark  its  final  and  unconditional  surrender. 

Adaptation  of  the  Organism  to  its  Environment.  We  can  dis- 
tinguish in  Pteris  as  clearly  as  in  Liimhincus  the  adaptation  of 
the  organism  to  its  environment.  The  aerial  part  of  Pteris 
must  be  fitted  to  make  exchanges  with,  and  maintain  its  life  in, 
the  atmosphere,  while  the  underground  part  must  be  similarly 
^ '  adapted  ' '  to  the  soil  in  wliich  it  lives. 

144 


ADAPTATION  TO   TRE  ENVIRONMENT.  145 

The  aerial  part  displays  admirable  adaptation  in  its  stalk,  w  liich 
rises  to  a  point  of  vantage  for  procuring  air  and  light,  and  in  its 
broadly  spreading  top,  which  is  covered  by  a  skin,  tough  and 
impervious,  to  prevent  undue  evaporation  and  consequent  desic- 
cation, yet  translucent,  to  allow  the  sun's  rays  to  reacli  the 
starch-making  tissue  within.  The  rhizome  also,  witli  its  pointed 
terminal  buds,  its  elongated  roots,  armed  with  Ijoring  tips,  and 
its  thick,  Heshy  parenchyma  for  the  storage  of  food,  is  admirably 
adapted  to  its  own  special  surroundings.  In  order  to  realize 
this,  we  have  only  to  imagine  the  fern  to  be  inverted,  the  aerial 
portion  being  planted  in  the  earth,  and  the  underground  portion 
lifted  into  the  air  and  exposed  to  the  winds  and  sunsliine.  Under 
these  circumstances  the  want  of  adaptation  of  tlie  parts  to  their 
respective  environments  would  speedily  become  apparent. 

Yet  different  as  these  parts  now  are,  tliey  have  originally 
sprung  from  the  same  cell.  More  recently  they  were  barely  dis- 
tinguishable in  a  mass  of  tissue,  part  of  which  turned  upwards 
into  the  air,  while  another  part  turned  downwards  into  the  earth. 
But  as  development  went  on,  the  aerial  and  underground  parts 
were  progressively  diiferentiated,  thus  becoming  more  and  more 
perfectly  adapted  to  the  peculiar  conditions  by  which  each  is 
surrounded. 

Thus  it  appears  that  the  harmony  between  every  part  of  the 
plant  and  its  environment  is  brought  about,  as  in  the  animal,  by  a 
gradual  process  in  the  history  of  each  individual.  We  can  here 
clearly  see  also  the  functional  adaptation  of  the  ])lant  to  chang- 
ing external  conditions.  The  environment  of  Pteris  changes 
periodically  with  the  regular  alternation  of  summer  and  winter, 
and  the  plant  also  undergoes  a  corresponding  periodic  change  of 
structure  in  order  to  maintain  its  adaptation  to  the  environment. 
During  the  summer  the  aerial  part  is  fully  developed,  and,  as  a 
result  of  its  activity,  starch  is  accumulated  in  the  rhizome.  At 
the  approach  of  winter  the  aerial  part  dies,  and  the  plant  is  re- 
duced to  the  underground  part  safely  buried  in  the  soil.  During 
the  winter  and  spring  the  starch  is  gradually  consumed,  and  the 
aerial  part  is  put  forth  again  as  the  aerial  environment  becomes 
once  more  favorable  to  it.  The  plant,  therefore,  like  the  animal, 
possesses  a  certain  plasticity  which  enables  it  to  adapt  itself  to 
gradually  changing  conditions  of  the  enviromnent. 


146  THE  BIOLOGY   OF  A   PLANT. 

A  little  consideration  will  show  that  every  function  or  action  of  living 
things  may  be  regarded  as  contributing  to  the  same  great  end,  viz.,  har- 
mony with  the  environment ;  and  from  this  point  of  view  life  itself  has 
been  defined  as  ' '  the  continuous  adjustment  of  internal  relations  to  ex- 
ternal relations^  * 

Nutrition.  The  fern  does  work.  In  pushing  its  stem 
through  the  soil,  in  lifting  its  leaves  into  the  air,  in  moving 
food-matters  from  point  to  point,  in  building  new  tissue,  in  the 
process  of  reproduction,  and  in  all  other  forms  of  vital  action, 
the  plant  expends  energy.  Here,  as  in  the  annual,  the  imme- 
diate source  of  energy  is  the  living  protoplasm,  which,  as  it 
lives,  breaks  down  into  simpler  compounds.  Hence  the  need  of 
an  income  to  supply  the  power  of  doing  work. 

The  Income.  The  income  of  the  fern,  like  that  of  the  earth- 
worm, is  of  two  kinds,  viz. ,  matter  and  energy,  but  unlike  that 
of  the  worm  it  is  not  chiefly  an  income  of  foods  ^  hut  only  of  the 
raw  'materials  of  food.  Matter  enters  the  plant  in  the  liquid  or 
gaseous  form  by  diffusion^  both  from  the  soil  through  the  roots 
(liquids),  and  from  the  atmosj)here  through  the  leaves  (gases). 
We  have  here  the  direct  absorption  into  the  body  j)roj)er  of  food- 
stuffs precisely  as  the  earthworm  takes  in  water  and  oxygen. 
Energy  enters  the  plant,  to  a  small  extent,  as  the  potential  energy 
of  food-stuffs,  but  comes  in  principally  as  the  kinetic  energy  of 
sunlight  absorbed  in  the  leaves.  The  table  on  p.  147  shows  the 
precise  nature  and  the  more  important  sources  of  the  income. 

Of  the  substances,  the  solids  (salts,  etc.)  must  be  dissolved 
in  water  before  they  can  be  taken  in.  Water  and  dissolved  salts 
continually  pass  by  diffusion  from  the  soil  into  the  roots,  where 
together  they  constitute  the  sap.  The  sap  travels  throughout 
the  whole  plant,  the  main  though  not  the  only  cause  of  move- 
ment being  the  constant  transpiration  (evaporation)  of  watery 
vapor  from  the  leaves,  especially  through  the  stomata.  The 
gaseous  matters  (carbon  dioxide,  oxygen,  nitrogen)  enter  the 
plant  mainly  by  diffusion  from  the  atmosphere,  are  dissolved  by 
the  sap  in  the  leaves  and  elsewhere,  and  thus  may  pass  to  every 
portion  of  the  plant. 

The  Manufacture  of  Foods — especially  Starch.  Pteris  owes 
its  power  of  absorbing  the  energy  of  sunlight  to  the  chlorophyll- 

*  Spencer,  Principles  of  Biology,  vol.  i.  p.  80.     N.  Y.,  Appleton,  1881. 


INCOME  OF  THE  PLANT. 


147 


hGclies  or  (Jiromatophores  j  for  plants  wliicli,  like  fungi,  etc.,  are 
devoid  of  chlorophyll  are  unable  thus  to  acquire  enei'gy.  Enter- 
ing the  chlorophyll-bodies,  the  kinetic  energy  of  sunlight  is  ap- 
plied to  the  decomposition  of  carbon  dioxide  (COJ  and  water 
(H^O).  Alter  passing  through  manifold  but  imperfectly  known 
processes,  the  elements  of  these  substances  linally  reappear  as 
starch  (CjIIj^OJ  often  m  the  form  of  granules  imbedded  in  the 
chloro^^hy  11 -bodies,  and  free  oxygen,  most  of  which  is  returned 


INCOME    OF    PTERIS. 


Matter. 

Whence  Derived. 

Carhon. 

Mainly  from  the  atmosphere  as  carbon  dioxide  (COq),  but  per- 
haps partly  from,  dissolved  organic  matters  (food). 

Hydrogen. 

Mainly  from  the  soil  as  water  (H2O),  but  perhaps  partly  from 
organic  foods. 

Oxygen. 

Mainly  from  the  soil  as  water  (H2O)  and  frona  the  air  as  free 
oxygen. 

Nitrogen. 

Mainly  from  the  soil  *  as  nitrates  or  ammonium  compounds,  or 
organic  foods. 

Sxdphur. 

Mainly  from  the  soil  as  sulphates. 

Other  elements. 

Mainly  from  the  soil  as  various  salts. 

Energy. 

Kinetic. 

Mainly  from  the  sunlight  through  the  leaves. 

Potential. 

Perhaps  to  a  limited  extent  in  food  materials  via  the  roots. 

to  the  atmosphere.     Thus  the  leaf  of  Pteris  in  the  light  is  con- 
tinually absorbing  carbon  dioxide  and  giving  forth  free  oxygen. 

Carbon  dioxide  and  water  contain  no  potential  energy,  since 
the  affinities  of  their  constituent  elements  are  completely  sat- 
isfied. Starch,  however,  contains  potential  energy,  since  the 
molecule  is  relatively  unstable,  i.e.,  capable  of  decomposition 
into  simpler,  stabler  molecules  in  which   stronger  affinities  are 

*  It  lias  been  generally  believed  that  plants  are  unable  to  make  use  of  free 
atmospheric  nitrogen,  but  recent  investigations  have  disproved  this  view  for 
certain  species. 


148  THE  BIOLOGY  OF  A  PLANT. 

satisfied.  And  this  is  due  to  tlie  fact  that  in  the  manufacture 
of  starch  in  the  chlorophyll-bodies  the  kinetic  energy  of  sunlight 
a  was  expended  in  lifting  the  atoms  into  position  of  vantage,' 
thus  endowing  them  with  energy  of  position.  In  this  way  some 
of  the  radiant  and  kinetic  energy  of  the  sun  comes  to  be  stored 
up  as  potential  energy  in  the  starch.  In  short,  Pteris^  like  all 
green  plants,  is  able  by  co-operation  with  sunlight  to  use  sunple 
raw  materials  (carbon  dioxide,  water,  oxygen,  etc.)  poor  in  en- 
ergy or  devoid  of  it,  and  out  of  them  to  manufacture  food  ^  i.e., 
complex  compounds  rich  in  available  potential  energy.  We 
shall  see  hereafter  that  this  power  is  possessed  by  green  plants 
alone ;  all  other  organisms  being  de23endent  for  energy  upon  the 
potential  energy  of  ready-made  food.  This  must  in  the  first 
instance  be  provided  for  them  by  green  plants ;  and  hence  with- 
out chlorophyll-l)earing  plants  animals  (and  colorless  plants  as 
well)  apparently  could  not  long  exist. 

The  plant  absorbs  also  a  small  amount  of  kinetic  energy,  in- 
dependently of  the  sunlight,  in  the  form  of  heat ;  this,  however, 
is  probably  not  a  source  of  vital  energy,  but  only  contributes  to 
the  maintenance  of  the  body  temperature. 

Circulation  of  Foods.  It  is  chiefly  in  the  green  (chloroj^hyll- 
bearing)  parts  of  the  plants,  and  in  the  presence  of  sunlight,  that 
food-manufacture  goes  on.  Somehow,  then,  the  water  absorbed 
by  the  roots  must  be  transported  to  the  leaves,  and  the  starch 
made  in  the  leaves  must  be  conveyed  to  the  subterranean  tissues. 
Exactly  how  these  transfers  of  material  are  effected  is  uncertain, 
but  there  is  reason  to  believe  that  they  take  place  mainly  by  the 
slow  processes  of  diffusion.  It  is  certain  that  no  distinct  organs 
of  circulation  or  distribution,  such  as  the  blood-vessels  of  the 
earthworm,  exist  in  the  fern. 

Metabolism.  Starch,  as  has  just  been  seen,  is  first  formed  in 
the  chlorophyll-bodies.  But  the  formation  of  starch,  all-imj)or- 
tant  as  it  is,  is  after  all  only  the  manufacture  of  food  as  a  j^re- 
liminary  to  the  real  processes  of  nutrition.  These  processes  must 
take  place  everywhere  in  ordinary  protoplasm;  for  it  is  here 
that  oxidations  occur  and  the  need  for  a  renewal  of  matter  and 
energy  consequently  arises  (cf.  pp.  32  and  33).  Sooner  or  later 
the  starch  grains  are  changed  into  a  kind  of  sugar  {glucose, 
CgHj^Og),  which,  unlike  starch,  dissolves  in   the  sap,  and  may 


OUTGO   OF  THE  PLANT.  149 

tlms  be  easily  transported  to  all  parts  of  the  plant.  Wherever 
there  is  need  for  new  protoplasm,  whether  to  repair  previous 
waste  or  to  supply  materials  for  growth,  after  absorption  into 
the  cells  the  elements  of  the  starch  (or  glucose)  are,  by  the  liv- 
ing protoplasm,  in  some  unknown  way  combined  with  nitrogen 
and  sulphur  (probably  also  with  salts,  water,  etc.),  to  form  proteid 
matter.  The  particles  of  this  newly-formed  coni])ound  are  incor- 
porated into  the  protoplasm  (by  "  intus-susception,"  p.  4)  and,  in 
some  way  at  present  shrouded  in  mystery,  are  endowed  w4th  the 
properties  of  life.  We  do  not  know  how  long  they  may  remain 
in  the  living  state,  but  sooner  or  later  they  are  oxidized,  and,  as  a 
result  of  tJie  oxidation,  that  energy  is  set  free  which  enables  the 
fern  to  do  work  and  prolong  its  existence.  The  oxidized  prod- 
ucts are  afterwards  eliminated  (excreted)  from  the  cells. 

If  a  larger  quantity  of  starch  is  formed  in  the  chlorophyll 
bodies  than  is  immediately  needed  by  the  protoplasm  for  pur- 
poses of  repair  or  growth,  it  may  be  re-converted  into  starch 
after  journeying  as  glucose  through  the  plant,  and  be  laid  down 
as  ' '  reserve  starch  * '  in  the  parenchyma  of  the  rhizome,  or  else- 
where. Apparently,  when  this  reserve  supply  is  iinally  needed 
at  any  point  in  the  plant,  it  is  again  changed  to  glucose  and  trans- 
ported thither.  It  is  'probable  that  new  leaves  and  new  tissues 
generally,  are  always  formed  in  part  from  this  reserve  starch, 
and  not  solely  from  newly-formed  starch. 

In  dealing  with  the  metabolism  of  the  fern  we  may  safely 
assume,  as  we  have  done  already  for  the  earthworm,  a  constructive 
phase  {andbolisiii)  and  a  destructive  phase  {hatabolism) ;  but 
these  terras  represent  merely  probable  events,  not  known  facts. 

The  Outgo.  The  outgo,  like  the  income,  is  of  two  kinds, 
matter  and  energy,  but  it  cannot  be  so  readily  tal)ulated. 

The  plant  suffers  annually  a  great  loss  both  of  matter  and  of 
potential  energy  in  the  production  of  spores  and  in  the  autumnal 
dying-down  of  the  fronds.  But  matter  also  leaves  the  plant 
daily  as  carbon  dioxide  (in  small  quantities),  water,  and  oxygen, 
both  by  diffusion  through  the  epidermis  and  by  transpiration 
through  the  stomata.  Strictly  speaking,  the  term  outgo  should 
be  restricted  to  the  output  of  matter  which  has  at  some  time 
actually  formed  a  part  of  the  living  protoplasm ;  hence  it  does 
not  apply  to  the  oxygen,  which  is  sinij^ly  given  off  in  the  manu- 


150 


THE  BIOLOGY  OF  A  PLANT. 


facture  of  starch,  or  to  the  bulk  of  the  water  of  evaporation, 
which  passes  straight  through  the  plant  without  undergoing  any 
chemical  change.  Energy  likewise  leaves  the  plant  continuously 
both  as  heat  and  in  the  doing  of  mechanical  worh^  both  of  which 
are  involved  in  every  vital  act. 

Respiration.  It  has  been  remarked  that  ui  the  light  (i.e., 
when  manufacturing  starch)  Pteris  takes  in  carbon  dioxide  and 
gives  off  free  oxygen.  But  if  the  plant  be  dejDrived  of  light,  as 
at  night,  the  reverse  is  true,  and  the  plant  takes  in  a  small 
amount  of  oxygen  and  gives  ofl"  a  corresponding  amount  of  car- 
bon dioxide.     This  latter  process  is  the  true  hreathing  or  resj>i- 


PTERIS   AQUILINA. 

(Balance-Sheet  of  Nutrition.) 

Income. 

^ 

Outgo. 

Matter. 

Matter. 

Foods, 

Carbon  dioxide. 

Inorganic  salts. 

W^ater, 

Carbon  dioxide. 

Excreted  substances. 

Water, 

Reproductive  germs, 

Free  oxygen. 

Leaves,  etc.. 

Free  oxygen  —  from  decomposition 

of  carbon  dioxide  in  light. 

Energy. 

Energy. 

Sunlight  absorbed  by  chlorophyll. 

AVork  performed. 

Potential  energy  in  foods. 

Heat. 

Potential  energy  in  cast-oflE  matters, 

reproductive  germs,  etc. 

Balance  in  favor  of  the  living  Pteris : 

Matter. 

Tissues,  protoplasm,  starch,  cellulose,  chlorophyll,  etc. 

Energy. 

Potential  energy  in  organic  matters. 

ration  of  the  plant,  and  it  must  not  be  confounded  with  that 
taking  in  of  carbon  dioxide  and  gi\dng  off  of  oxygen  which  is  an 
incident  in  the  manufacture  of  starch.  Resj^iration  goes  on  in 
the  light  also,  probably  with  greater  energy  than  in  darkness, 
but  it  is  then  largely  obscured  by  the  other  and  more  conspicu- 
ous process.  We  have  seen  that  energy  is  set  free  in  living  mat- 
ter by  a  decomposition  of  its  own  substance,  which  is  really  a 
process  of  oxidation  or  combustion,  where  free  oxygen  plays 
an  important  part  (p.  32,  Chap.  III.) ;  hence  the  absorption  of 
free  oxygen  in  respiration.  Among  the  products  of  the  combus- 
tion, water  and  carbon  dioxide  are  the  most  im^^ortant ;   and  this 


ACTION   UPON  THE  ENVIRONMENT.  151 

is  the  origin  of  tlie  carbon  dioxide  given  oft".  It  will  appear 
beyond  that  precisely  the  same  action  takes  place  in  the  respi- 
ration of  animals,  and  that  all  living  things  breathe  or  respire  in 
essentially  the  same  way. 

It  was  for  a  long  time  believed  that  a  leading  difference  between  plants 
and  animals  lay  in  the  fact  that  the  former  give  off  oxygen  and  absorb 
carbon  dioxide,  while  the  latter  give  off  carbon  dioxide  and  absorb  oxygen. 
But  it  is  now  known  that  both  give  off  carbon  dioxide  and  both  require 
oxygen,  and  that  only  the  chlorophyll-bearing  parts  of  green  plants  are  en- 
dowed with  the  special  function  of  decomposing  carbon  dioxide  and  water 
and  manufacturing  starch — as  a  result  of  which  they  do  (but  in  tlie  light 
only)  give  off  oxygen  as  a  kind  of  incidental-  or  by-product. 

Interaction  of  the  Fern  and  its  Environment. 

The  actions  of  the  environment  upon  the  fern  have  already 
been  sufticiently  dwelt  upon  (p.  144).  It  still  remains,  however, 
to  consider  the  actions  of  the  fern  upon  the  environment. 
These  are  partly  physical,  but  mainly  chemical.  By  pushing 
its  fronds  into  the  air  and  slowly  thrusting  its  rhizome,  roots,  and 
branches  through  the  soil,  the  atmosphere  and  the  earth  are  alike 
displaced.  But  it  is  by  its  chemical  activity  that  it  most  pro- 
foundly affects  its  environment.  Absorbing  from  the  latter 
water,  salts,  carbon  dioxide,  and  other  simple  substances,  as  well 
as  sunlight,  it  produces  with  them  a  remarkable  metamorphosis. 
It  manufactures  from  them  as  raw  materials  organic  matter  in 
the  shape  of  starch,  fats,  and  even  proteids.  These  it  gives 
back  to  the  environment  in  some  measure  during  life,  and  sur- 
renders wholly  after  sudden  death.  But  the  most  striking  fact 
is  that  the  fern  is  on  the  whole  constnictive  and  capable  of  pro- 
ducing and  accumulating  compounds  rich  in  energy.  In  this 
respect  it  is  unlike  the  earthworm  (p.  lO-t)  and  is  typical  of  green 
plants  in  general.  Thus,  wdiile  animals  are  destroyers  of  ener- 
gized compounds,  green  plants  are  producers  of  them.  Ani- 
mals, therefore,  in  the  long  run  are  absolutely  dependent  on 
plants ;  and  animals  and  colorless  plants  alike  upon  green  plants. 
But  it  must  never  be  forgotten  that  most  plants  are  enabled  to 
manufacture  organic  from  inorganic  matter  by  virtue  of  the 
chlorophyll  which  they  contain.  Without  this  they  are  j^ower- 
less  in  this  respect.      (See,  however,  p.  11>T). 


152  THE  BIOLOGY  OF  A  PLANT. 

Physiology  of  the  Tissue- Systems.  The  ej)idermal  tissues 
serve  as  the  sole  medium  of  exchange  betweeu  the  mner  parts  of 
the  plant  and  the  envnronment ;  they  are  also  protective,  and  in 
certain  regions  are  useful  for  suj^port.  The  function  of  repro- 
duction also  falls  upon  these  tissues,  as  is  shown  by  the  develop- 
ment of  the  sporangia,  antheridia,  and  archegonia. 

Tlie  Jihro-vascidar  tissues  serve  in  part  as  a  supporting 
skeleton,  for  which  function  their  richness  in  prosencliyma 
and  their  Urm  continuity  admirably  adapt  them.  An  equally 
important  function,  however,  is  their  conductivity.,  since  they 
serve  for  the  transportation  of  the  water  for  evaporation  by  the 
leaf  {transpiration)^  and  for  the  movement  (through  the  sieve- 
tubes)  of  the  undissolved  and  indiffusible  proteids.  T\\q  funda- 
mental tissues  are  devoted  either  to  sharing  the  sjDccial  duties 
of  the  other  systems,  as  in  the  case  of  the  sclerotic  j^arenchyma 
abutting  upon  tlie  epidermal  tissue  in  the  rhizome  (p.  119),  and 
the  sclerotic  prosenchyma  which  appears  to  behave  like  the  fibro- 
vascular  tissues ;  or  to  nutritive  and  metabolic  functions,  as  in 
the  mesophyll  (p.  126)  and  the  parenchyma  of  the  rhizojne. 

The  Physiology  of  Reproduction.  It  is  not  known  whether  the 
brake  ever  dies  of  old  age.  Barring  accidents,  growth  at  the 
apical  buds  seems  to  be  unlimited,  keeping  pace  with  death  of 
the  hinder  parts  of  the  rhizome  (p.  111).  But  whether  the  indi- 
vidual dies  or  not,  ample  provision  against  the  death  of  the  race 
is  made  in  the  act  of  reproduction.  Although  reproduction  ap- 
pears to  be  useless  to  the  individual,  and  even  entails  u^^on  it 
serious  annual  losses  of  matter  and  energy,  yet  to  tliis  function 
every  part  of  the  plant  directly  or  indirectly  contributes.  The 
reproductive  germs  are  carefully  prepared;  are  provided  with  a 
stock  of  food  sufficient  for  the  earliest  stages  of  development ; 
and  are  endowed  w4th  the  peculiar  powers  and  limitations  of 
Pteris  aquilina.^  which  influence  their  life-history  at  every  step 
and  are  by  them  transmitted  in  turn  to  their  descendants.  They 
are  living  portions  of  the  parent  detached  for  rej)roductive  ^^ur- 
poses;  they  contain  a  share  of  protoj^lasm  directly  descended 
from  the  original  protoplasm  of  the  spore  from  which  the  parent 
came ;  and  thus  they  serve  to  efl" ect  that  ' '  continuity  of  the 
germ-plasm ' '  to  which  we  have  already  referred  in  dealing 
with  the   earthworm.      In   short,   reproduction  is  the  supreme 


PLANT  AND  ANIMAL   COMPARED.  lo3 

function  of  the  plant.  If  we  may  paraplirase  the  words  of 
Michael  Foster,  the  oosphere  is  the  goal  of  individual  existence, 
and  life  is  a  cycle,  beginning  with  the  oosphere  and  continually 
coming  round  to  it  again. 

Comparison  of  the  Fern  and  the  Earthworm.  To  the  super- 
ficial observer  the  fern  and  earthworm  seem  to  have  little  or 
nothing  in  common,  except  that  both  are  what  we  call  alive.  But 
whoever  has  studied  the  preceding  pages  must  have  perceived 
beneath  manifold  differences  of  detail  a  fundamental  likeness 
between  the  plant  and  animal,  not  oidy  in  the  substantial  iden- 
tity of  the  living  matter  in  the  two  but  also  in  the  construction 
of  their  bodies  and  in  the  processes  by  which  they  come  into 
existence.  Each  arises  from  a  single  cell  which  is  the  result  of 
the  union  of  two  differently-constituted  cells,  male  and  female. 
In  both  the  primary  cell  multiplies  and  forms  a  mass  of  cells,  at 
iirst  nearly  similar  but  afterwards  dift'erentiated  in  various  di- 
rections to  enable  them  to  perform  different  functions,  i.e.,  to 
effect  a  physiological  division  of  labor.  In  both,  the  tissues  thus 
provided  are  associated  more  or  less  closely  into  distinct  organs 
and  systems,  among  which  the  various  operations  of  the  body 
are  distributed.  And  in  both  the  ultimate  o^oal  of  individual 
existence  is  the  production  of  germ-cells  which  form  the  start- 
ing-point of  new  and  similar  cycles. 

This  fundamental  likeness  extends  also  to  most  of  the  actions 
(physiology)  of  the  two  organisms.  Both  possess  the  power  of 
adapting  themselves  to  the  environments  in  which  they  live. 
Both  take  in  various  forms  of  matter  and  energy  from  the  en- 
vironment, build  them  up  into  their  own  living  sul)stance,  and 
finally  break  down  this  substance  more  or  less  completely  into 
simpler  compounds  by  processes  of  internal  combustion,  setting 
free  by  this  action  the  energy  which  maintains  their  vital  ac- 
tivity. And,  sooner  or  later,  both  give  back  to  the  environment 
the  matter  and  energy  which  they  have  taken  from  it.  In  other 
words,  both  effect  an  exchanti^e  of  matter  and  of  enertj^v  with 
the  environment. 

Nevertheless  the  plant  and  the  animal  differ.  They  differ 
widely  in  form,  and  the  plant  is  fixed  and  relatively  rigid,  while 
the  animal  is  flexible  and  mobile.  The  body  of  the  plant  is 
relatively  solid;   that  of  the  animal  contains  numerous  cavities. 


154  THE  BIOLOGY  OF  A  PLANT. 

The  plant  absorbs  matter  directly  tlirougli  tlie  external  surface ; 
the  animal  partly  throngh  the  external  and  partly  through  an 
internal  (alimentary)  surface.  The  plant  is  able  to  absorb  simple 
chemical  compounds  from  the  air  and  earth,  and  kinetic  energy 
from  sunlight;  the  animal  absorbs,  for  the  most  part,  complex 
chemical  compounds  and  makes  no  nutritive  use  of  the  sun's 
kinetic  energy.  By  the  aid  of  this  energy  the  plant  manufac- 
tures starch  from  simple  compounds,  carbon  dioxide,  and  water ; 
the  animal  lacks  this  power.  The  j^lant  can  build  up  proteids 
from  the  nitrogenous  and  other  compounds  of  its  food ;  the  animal 
absolutely  requires  proteids  in  its  food.  And  by  manufacturing 
proteids  within  its  living  substance,  the  plant  is  relieved  of  the 
necessity  of  carrying  on  a  process  of  digestion  in  order  to  render 
them  diffusible  for  entrance  into  the  body. 

Still,  great  as  these  differences  appear  to  be  at  first  sight, 
all  of  them,  with  a  single  exception,  fade  away  upon  closer  ex- 
amination. This  exception  is  the  power  of  wiaking  foods. 
Plants  and  animals  differ  in  form  because  their  mode  of  life 
differs ;  but  a  wider  study  of  biology  reveals  the  existence  of  in- 
numerable animals  (corals,  sponges,  hydroids,  etc.)  which  have 
a  close  superficial  resemblance  to  plants,  and  of  many  plants 
which  resemble  animals,  not  only  in  form,  but  also  in  possessing 
the  power  of  active  locomotion.  The  stomach  of  the  worm,  as 
shown  by  its  development,  is  really  a  part  of  the  general  outer 
surface  which  is  folded  into  the  body ;  and  the  animal,  like  the 
plant,  therefore,  really  absorbs  its  income  over  its  whole  surface 
— oxygen  through  the  general  outer  surface,  other  food-matters 
through  the  infolded  alimentary  surface. 

In  like  manner  it  is  easy  to  show^  that  not  one  of  the  differ- 
ences between  the  plant  and  animal  is  fundamentally  impor- 
tant save  the  jpower  of  inaMiig  foods.  The  worm  must  have 
complex  ready-made  food  including  proteid  matter.  So  must 
the  fern ;  but  the  fern  is  able  to  mamfacture  this  complex  food 
out  of  very  simple  compounds.  In  terms  of  energy,  the  worm 
requires  ready-made  food  rich  in  potential  energy;  the  fern, 
aided  by  the  sun's  energy,  can  manufacture  food  from  matters 
devoid  of  energy. 

Hence  it  appears,  broadly  speaking,  that  the  fern  by  the  aid 
of  solar  energy  is  constructive,  and  stores  up  energy ;   the  earth- 


FOOD   OF  PLANTS  AND  ANIMALS.  155 

worm  is  destructive,  and  dissipates  energy.  And  this  difference 
becomes  of  immense  importance  in  view  of  the  fact  that  the 
fern  is  typical  in  this  respect  of  all  green  plants,  as  the  earth- 
worm is  typical  of  all  animals. 

It  will  hereafter  api)ear  that  even  this  difference,  great  as  it 
is,  is  partly  bridged  over  by  colorless  plants  like  yeast,  nionlds, 
bacteria,  etc.,  which  have  no  chlorophyll,  are  therefore  unable 
to  use  the  energy  of  light,  and  hence  must  have  energized  fo(>d. 
But  these  organisms  do  not,  like  animals,  require  proteid  food, 
being  able  to  extract  all  needful  energy  from  the  simpler  fats, 
carbohydrates,  and  even  from  certain  salts.  When  we  consider 
that  the  distinctive  peculiarities  of  animals  can  thus  be  reduces  I 
to  the  sole  characteristic  of  dependence  on  proteid  food,  we  can- 
not doubt  that  the  differences  between  plants  and  animals  are  of 
immeasurably  less  importance  than  their  fundamental  likeness. 


It  has  been  the  object  of  the  foregoing  chapters  to  give  the 
student  a  general  conception  of  organisms,  whether  vegetal  or 
animal ;  of  their  structure,  growth,  and  mode  of  action  ;  of  their 
position  in  the  world  of  matter  and  energy,  and  of  their  relations 
to  lifeless  things.  With  this  preliminary  knowledge  as  a  basis, 
the  student  is  prepared  to  take  up  the  progressive  study  of  other 
organisms,  selected  as  convenient  types  or  examples.  It  is  con- 
venient to  begin  with  low  and  simple  forms  of  life  and  work 
gradually  upwards;  and  it  is  especially  desirable  to  do  so  be 
cause  there  is  reason  to  believe  that  this  course  corresponds 
broadly  with  the  path  of  actual  evolution. 


CHAPTER  XL 

THE  UNICELLULAR   ORGANISMS. 

It  lias  been  shown  in  the  foregoing  pages  that  the  complex 
body  of  an  adult  fern  or  earthworm,  or  of  any  of  the  higher 
forms  of  life,  originates  from  a  single  cell  of  microscopic  size. 
This  cell — the  fertilized  ovum  or  oosphcre — gives  rise  by  divi- 
sion to  new  cells  which  in  their  turn  divide,  generation  after 
generation,  until  a  full-grown  hody  is  formed,  composed  of 
myriads  of  cells.  But  the  process  of  cell-division  does  not  in 
this  case  go  as  far  as  complete  o,^- separation^  and  the  cells  do 
not  acquire  a  complete  individuality.  They  do,  it  is  true,  ac- 
quire a  certain  independence  of  structure  and  function;  and 
their  individual  characteristics  may  even  dej)art  widely  from 
those  of  neighboring  cells  (differentiation).  Nevertheless  they 
remain  closely  united  by  either  material  or  physiological  bonds  to 
form  one  body.  The  body  is  not,  however,  to  be  regarded  as 
merely  an  assemblage  of  independent  individual  cells.  The  hody 
is  the  individual  ^  its  more  or  less  perfect  division  into  cells  is 
only  a  basis  for  the  physiological  division  of  labor;  of  which 
cell-differentiation  is  the  outward  exjDression. 

All  this  is  true,  howe^'er,  only  in  the  higher  types.  At  the 
bottom  of  the  scale  of  life  there  is  a  vast  multitude  of  forms  in 
which  the  body  consists,  not  of  many  cells  but  of  only  one,  and  is 
therefore  comparable  in  structure  not  to  the  adult  fern  or  earth- 
worm, but  to  the  germ-cells  from  which  these  arise.  Such  forms 
are  known  as  unicellular  organisms,  in  contradistinction  to  the 
Tnulticellular.  Like  other  cells  the  unicellular  organisms  multi- 
ply by  division,  but  division  is  followed  sooner  or  later  by  com- 
plete separation  ;  the  daughter-cells  become  entirely  distinct  and 
independent  individuals,  and  do  not  remain  permanently  asso- 
ciated. In  them  a  true  multicellular  body,  therefore,  is  never 
formed ;   the  cell  is  the  individual^  and  the  hody  is  unicelhdar. 

156 


THE   UNICELLULAR  BODY.  157 

Nevertheless  the  one-celled  organism  perforins  all  of  the 
characteristic  operations  of  life.  A  single  mass  of  protoplasm,  a 
single  cell,  unites  in  itself  the  performance  of  all  the  various 
elementary  functions  which  in  the  multicellular  forms  are  distrib- 
uted among  many  cells,  differentiated  into  divers  tissues  and 
organs.  The  unicellular  forms  are  therefore  in  a  physiological 
sense  as  truly  ' '  organisms ' '  as  the  multicellular  forms ;  and  in 
many  cases  the  unicellular  body  shows  a  very  considerable  degree 
of  differentiation  among  its  parts.  But  the  unicellular  forms 
are  organisms  reduced  to  their  lowest  terms ;  they  present  us  with 
the  problems  of  life  in  their  most  rudimentary  form.  Hence 
they  may  afford  a  kind  of  key  to  the  more  elaborate  organization 
of  the  higher  types. 

We  shall  find  among  unicellular  forms  representatives  both 
of  animals  and  of  plants,  and  to  a  detailed  examination  of  some 
of  these  we  may  now  proceed. 


// 


CHAPTER  XII. 

UNICELLULAR  ANIMALS   {Protozoa). 

A.  Amoeba. 

(Tlie  Proteus  Animalcule.) 

General  Account.  Amoeba  is  a  minute  organism  occasionally 
found  in  stagnant  water,  in  the  sediment  at  the  bottom  of  ponds 
and  ditches,  on  the  surface  of  water-plants,  in  damp  earth,  in 
organic  infusions  of  various  kinds — almost  anywhere,  in  short,  in 
the  presence  of  moisture,  organic  matter,  and  other  favorable 
conditions.  There  are  many  species  of  Amoeba,  some  living  in 
salt  water,  others  in  fresh.  One  of  the  largest  and  commonest 
fresh-water  forms  is  A7noeba  Proteus,  which  forms  the  subject 
of  this  account.* 

Am^oeba  occurs  in  an  active  or  m^otile  state,  and  a  quiescent  or 
encysted  state.  When  active  the  body  consists  (Fig.  84)  of  a 
minute  naked  mass  of  protoplasm  which  in  the  case  of  large 
specimens  is  barely  visible  to  the  naked  eye — i.e.,  half  a  milli- 
metre ("5^0  inch)  or  less  in  length.  This  mass  creeps,  or  rather 
flows,  actively  about  by  the  continual  protrusion  of  lobes  or  proc- 
esses of  its  own  substance,  known  as  jpseudopodia.  These  may 
be  put  forth  from  any  part  of  the  surface  and  again  merged  into 
the  general  mass;  the  body  therefore  continually  changes  its 
shape,  and  hence  the  name  ' '  Proteus. ' ' 

When  the  body  is  well  extended  the  protoplasm  is  seen  to 
consist  of  a  clear  peripheral  substance,  the  ectoplasm,  and  a  cen- 
tral substance,  the  entoplasm,  filled  with  coarse  granules  which 
give  the  body  a  highly  characteristic  granular  appearance  some- 
times described  as  a  ''gray  color."  Within  the  ectoplasm  the 
more  fluid  entoplasm  freely  flows,  as  if  confined  in  a  tube  or 

*  Other  common  forms  are  the  smaller  A.  radiosa  and  A.  verrucosa.  The 
large  A.  {Pelomyxa)  mllosa  and  A  {Dinamceba)  mirabilis  are  not  infrequent. 
See  Leidy,  Fresh-water  Rhizopods  of  North  America. 

158 


THE  PROTEUS  ANIMALCULE. 


159 


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V:;:?vi''2.6;i^r:.V<j:rr-f 


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v. 


.«»'■ 


V — , — '. 


rvri/-     f.v    i 


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Fig.  H.—Amccba  Proteus,  from  life  X  300,  The  arrows  indicate  the  direction  of  the 
protoplasmic  currents;  ?»,  nucleus;  c.v,  contractile  vacuole;  f.v,  food-vacuole ; 
w.v,  water-vacuole.  A  shows  the  texture  of  the  protoplasm.  B  is  an  outline  of 
the  same  individual  four  minutes  later ;  the  upward  currents  at  the  right  of  Fig. 
A  have  stopped,  reversed,  and  the  main  flow  is  now  towards  the  left. 


160  UNICELLULAR  ANIMALS. 

sac,  but  tlie  two  substances  are  not  separated  hj  any  definite 
boundary-line,  and  pass  imperceptibly  into  one  another.  The 
external  boundary  of  the  body  is  formed  by  the  outermost  limit 
of  the  ectoplasm.  There  is  no  membrane,  and  the  body  is 
quite  naked.  ^N^evertheless  the  protoplasmic  mass  shows  no 
tendency  to  mix  with  the  surrounding  water,  and  perfectly  main- 
tains its  integrity ;   it  is  an  individual. 

The  formation  of  a  pseudopod  begins  by  the  bulging  out  of 
the  ectoplasm  to  form  a  rounded  prominence  at  some  point  on 
the  surface.  Into  its  interior  a  sudden  gush  of  entoplasm  then 
takes  place  and  a  steady  outward  stream  ensues,  the  entoplasm 
pushing  the  ectoplasm  before  it,  and  the  substance  of  the  body 
flowing  into  the  pseudopod.  The  whole  substance  of  the  body 
may  thus  flow  onward  into  the  pseudopod,  which  meanwhile  forms 
new  pseudopods,  and  so  the  entire  animal  advances  in  the  direction 
of  the  flow ;  or,  the  pseudopod  after  attaining  a  certain  size  may 
be  withdrav\Ti  into  the  body  by  reverse  (centripetal)  currents,  the 
main  body  having  meanwhile  flowed  onward  in  another  direction. 

As  a  rule,  the  new  pseudopodia  are  put  forth  near  one  end 
of  the  body  (hence  called  "  anterior  "),  and  the  general  direction 
of  advance  is  therefore  fairly  constant,  not  vague  and  indefinite, 
as  is  often  stated.  The  direction  of  flow  fluctuates,  however, 
about  a  certain  mean,  being  continually  diverted  this  way  or 
that  by  the  formation  of  new  pseudopodia.  Those  which  do  not 
form  directly  in  the  line  of  march  either  merge  little  by  little 
with  the  advancing  ones,  or  are  withdrawn  by  reversed  currents 
into  the  body.  In  the  latter  case  they  often  leave  shrivelled 
wart-like  remnants,  and  a  group  of  similar  warts  is  usually 
found  near  the  "j)osterior"  end  of  the  body  (Fig.  84,  j^)- 
Definite  changes  in  the  general  du'ection  of  advance  are  effected 
by  the  diversion  of  the  main  current  into  lateral  pseudopodia. 

Amoeba  feeds  upon  minute  plants  and  animals  or  other  or- 
ganic particles.  There  is  no  mouth,  and  food -matters  are  bodily 
ingulfed  (at  no  definite  point)  by  the  proto]3lasm  which  closes 
up  beyond  them.*     The  indigestible  remains  are  passed  out  in 

*  This  mode  of  cellular  alimentation  is  of  frequent  occurrence  in  some  cells 
of  multicellular,  as  well  as  in  unicellular,  animals.  Cells  exhibiting  it  are 
known  as  phagocytes  (eating-cells),  and  the  process  is  referred  to  as  phagocytosis. 
It  is  obviously  only  a  prelude  to  intra-cellular  digestion. 


ENCYSTED  STATE  OF  AMCEBA. 


161 


an  equally  primitive  fashion,  nsually  at  some  point  near  the 
' '  posterior  ' '  end.  Besides  solid  food-stuffs  uhnaiha  takes  in  a 
certain  quantity  of  water  (along  with  minute  quantities  of  inor- 
ganic salts  dissolved  in  it),  and  it  also  breathes,  by  taking  in 
(mainly  by  ditfusion)  tlie  free  oxygen  dissolved  in  the  water  and 
giving  off  carbon  dioxide. 

Such  is  Ammha  in  its  active  phase.  The  quiescent  or  en- 
cysted  state  is  entered  upon  under  conditions  not  thoroughly 
understood,  but  probably  of  an  unfavorable  nature,  such  as  the 


V -^o; -St .•.■•••  .^rr^si 


Fig.  85.— vl,  Amccba  dividing  by  fission,  nucleus  not  seen  (after  Leidy).  C,  Amccha 
after  a  full  meal  consisting  of  a  large  diatom  (dt).  (After  Leidy).  Letters  as  in 
Fig.  84.    D,  Encysted  Amoeba,  containing  food-matters  (after  Howes). 

lack  of  food,  drying  up  of  ponds,  and  the  like.  The  pseudo- 
podia  are  withdrawn,  movement  ceases,  the  body  becomes 
spherical  and  surrounds  itself  with  a  tough  membrane  (cell- wall) 
(Fig.  85,  D).  The  animal  takes  no  food  and  all  of  its  activities 
are  nearly  suspended.  It  is  like  an  animal  asleep  or  hibernating, 
and  in  this  state  it  may  long  remain.  Protected  by  its  mem- 
brane it  is  able  to  resist  desiccation,  and  upon  the  evaporation  of 
the  surrounding  water  it  may,  as  a  particle  of  "  dust,"  be  trans- 
ported by  the  winds,  even  to  a  great  distance.  When  again 
placed  under  favorable  conditions  the  protoplasm  bursts  its 
envelope,  crawls  forth  from  it,  and  reassumes  its  active  phase. 


162  UNICELLULAR  ANIMALS. 

Structure.  Lying  in  the  entoplasm,  usually  near  the  pos- 
terior extremity,  is  a  nucleus  {n^  Fig.  84),  having  the  form  of 
a  bi-concave  disk  and  largely  made  up  of  coarse  granules  of 
cTiTomatin  (cf.  p.  23).  Amoeba  is  therefore  at  once  a  single 
cell  and  a  unicellular  organism,  morphologically  equivalent  to  a 
single  tissue-cell  of  a  higher  animal  or  to  the  germ-cell  from 
which  every  multicellular  form  arises.  The  hody  of  Ammha  is 
a  one-celled  body. 

The  protoplasTii  (cytoplasm)  consists  of  a  clear  basis,  and  (in 
the  case  of  the  entoplasm)  of  innumerable  granules  extremely 
diverse  in  form  and  size,  and  frequently  differing  in  character 
in  different  individuals.  Often  thev  are  in  the  form  of  rhom- 
boidal  crystalline  bodies;  in  other  cases  they  are  rounded  or 
irregular.  Their  precise  chemical  composition  is  uncertain,  but 
they  are  probably  comj^lex  organic  compounds,  a  product  of 
metabolism  and  serving  as  reserve  food-matter.* 

Vacuoles.  The  protoplasm  often  contains  rounded  vacuoles 
of  which  the  three  following  kinds  may  be  distinguished  : 

{ft)  Water -vacuoles  {w.v,  Figs.  84,  85),  filled  with  water, 
lying  in  the  entoplasm  and  carried  along  in  its  currents. 

(h)  jFood -vacuoles  {f.v)^  also  lying  in  the  entoplasm,  con- 
taining the  solid  food-matters  that  have  been  ingulfed.  Within 
them  digestion  takes  place.  When  this  process  is  completed 
they  approach  the  exterior — usually  at  some  point  near  the 
posterior  end — the  outer  wall  breaks  through,  and  the  innutri- 
tions remnants  are  cast  out,  the  ectoplasm  closing  up  the  breach 
immediately  afterwards.  Thus  Amoeba  has  no  mouth,  ali- 
mentary canal,  or  anus,  but  the  general  mass  of  protoplasm 
plays  the  role  of  all  three. 

{c)  Contractile  vacuole  (c.v).  Usually  single,  sometimes 
double,  lying  near  the  posterior  end,  and  filled  with  liquid. 
This  is  sharply  distinguished  from  the  other  vacuoles  by  its 
rhythmical  pulsation,  expanding  (diastole)  and  contracting  {sys- 
tole) at  regular  intervals.  During  the  diastole  the  vacuole  slowly 
fills  with  liquid  which  drains  into  it  from  the  surrounding  j)roto- 
plasm.  At  the  systole,  which  is  very  sudden,  tliis  liquid  is  forci- 
bly expelled  to  the  exterior  through  an  opening  that  breaks 

*  In  some  species  of  Amoeba  the  entoplasm  may  also  contain  innumerable 
grains  of  sand  taken  in  from  the  exterior,  but  this  is  not  the  case  in  A.  Proteus. 


PHYSIOLOGY  OF  AMCEBA.  163 

through  the  ectoplasm,  and  immediately  afterwards  disappears. 
The  contractile  vacuole  is  almost  certainly  to  be  regarded  as  a 
simple  kind  of  excretory  apparatus,  the  water  which  collects  in 
it  containing  in  solution  various  products  of  destructive  metabo- 
lism which  are  thus  passed  out  of  the  body.* 

Reproduction.  However  abundant  the  food-supply  Amceha 
never  grows  beyond  a  certain  maximum  limit.  x\fter  this  limit 
has  been  attained  the  animal  sooner  or  later  divides  by  '^ fission'*'' 
into  two  smaller  AmoebcB  (Fig.  85,  A).  Thus  the  existence  of 
an  individual  Amoeha  is  normally  terminated,  not  by  death,  but 
by  resolution  into  two  new  individuals.  This  process  is  the 
simplest  possible  form  of  agamogenesis,  and  Amoeba  is  not  known 
to  multiply  in  any  other  way.f  The  fission  of  Amoeha  is  a 
process  essentially  of  the  same  nature  as  the  division  of  ordinary 
tissue-cells,  a  division  of  the  nucleus  preceding  that  of  the 
cytoplasm.  Whether  the  division  of  the  nucleus  is  of  tlie  indi- 
rect type  (i.  e. ,  passes  through  the  phenomena  of  karyokinesis)  is 
not  known  by  direct  observation,  but  there  is  some  reason  to  be- 
lieve that  it  is  so.  In  any  case  the  successive  fissions  of  Amoeba 
are  directly  comparable  with  the  successive  cleavages  of  the  e^^ 
of  a  metazoon  (p.  25).  Tlie  progeny  of  the  Amoeba^  however, 
separate  and  form  independent  individuals,  while  those  of  the  egg- 
cell  remain  intimately  associated  to  form  a  single  multi-cellular 
individual.  Morphologically,  therefore,  a  metazoon  is  comparable 
not  with  a  single  Amoeba^  but  mth  a  multitude  of  Amoebo^. 

Physiology.  The  possible  simplicity  of  animal  structure  is 
well  shown  in  Atnoeba^  which  is  morphologically  an  animal  re- 
duced to  its  lowest  terms.  Its  physiological  ojierations  are  cor- 
respondingly primitive  and  rudimentary ;  and  by  an  analysis  of 
them  we  may  discover  what  is  essential  and  fundamental  in  the 
physiology  of  animals  in  general.  A  survey  of  the  various  activ- 
ities of  Amoeba  shows  that  these  may  all  be  reduced  toaiewfunda- 
inental ])liysiological  j^y^oioerties  of  tlie  protoplasm, ;[:  as  follows: 

*  It  may  be  recalled  that  the  cavity  of  the  nephridium  in  the  earthworm  is 
intra-cellular,  like  a  vacuole  (p.  60). 

f  It  has  been  asserted  that  Amceba  conjugates  and  also  that  it  multiplies  by 
endogenous  division  ;  but  the  evidence  on  both  these  points  is  inconclusive, 

X  It  is  hardly  necessary  to  remark  that  in  common  with  all  English-speak- 
ing biologists  we  are  indebted  to  Foster  for  the  first  comprehensive  elaboration 
of  the  "  fundamental  physiological  properties  "  as  exhibited  by  A7naba. 


164  UNICELLULAR  ANIMALS. 

(1)  Contractility^  by  means  of  which  motion  is  effected. 
This  appears  most  clearly  when  the  animal  is  stimulated  by  a 
sudden  jar,  or  by  an  electric  shock,  which  causes  the  body  to 
contract  into  a  ball.  This  property,  precisely  like  the  contraction 
of  a  muscle  (p.  27),  is  the  result  of  a  molecular  rearrangement, 
accompanied  by  chemical  changes,  which  causes  a  change  of 
form  in  the  mass  without  altering  its  bulk.  The  action  of  the 
contractile  vacuole  is  due  to  the  contractility  of  the  surrounding 
protoplasm ;  and  in  like  manner  the  currents  which  cause  the 
protrusion  and  withdrawal  of  pseudopods,  and  so  the  locomotion 
of  the  animal  as  a  whole,  are  produced  by  localized  contractions 
of  the  i^eriplieral  layer  of  protoplasm  which  drive  onwards  the 
more  fluid  central  parts. 

(2)  IrritaMlity  {including  Co-ordination)^  or  the  power  to 
be  afliected  by,  and  to  respond  to,  changes  or  ' '  stimuli ' '  acting 
upon  or  within  the  protoplasm.  The  change  of  shape  following 
the  application  of  an  electric  shock  is  actually  effected  by  con- 
tractility, but  the  power  to  be  affected  by  the  shock  and  to  arouse 
contractility,  is  irritability.  To  this  property  the  animal  owes  its 
power  of  performing  adaptive  actions  in  response  to  changes  in 
the  environment,  and  also  its  power  to  co-ordinate  the  various 
actions  of  its  own  body.  To  illustrate  :  It  is  a  remarkable  fact 
that  Amoeba  is  able  to  discriminate  between  nutritious  and  innu- 
tritious  matters,  ingulfing  the  former,  but  rejecting  the  latter. 
Physiologically  this  discrimination  is  a  difference  of  response  to 
different  stimuli — hence  a  phenomenon  of  irritability.  Again, 
the  various  actions  (movements,  etc.)  of  Amceba^  despite  their 
apparently  vague  character,  are  co-ordinated  to  form  a  definite 
whole ;  and  co-ordination  may  be  regarded  as  a  phenomenon  of 
irritability,  changes  in  one  part  serving  as  stimuli  to  other  parts 
and  being  brought  into  orderly  relation  with  them.  The  property 
of  irritability  lies  at  the  base  of  all  nervous  activity  in  higher 
forms  (cf.  p.  67)  and  is  concerned  in  many  other  actions. 

(3)  Metdbolism^  the  most  fundamental  of  all  vital  actions, 
since  it  lies  at  the  root  of  all,  is  the  power  of  waste  and  repair — 
the  destructive  chemical  changes  in  protoplasm  {katabolism) 
whereby  energy  is  set  free,  and  the  constructive  actions  (anabo- 
lisrri)  through  which  new  protoplasm  is  built  and  potential 
energy  is  stored  (cf.  p.  33).     There  is  every  reason  to  believe 


PHYSIOLOGICAL  PROPERTIES  OF  AM(EBA  165 

that  the  metabolic  phenomena  of  Amoeba  are,  broadly  speaking, 
similar  to  those  of  higher  animals.  The  katabulic  changes  are  in 
the  long  run  processes  of  oxidation,  and  although  their  products 
have  not  yet  been  definitely  ascertained  in  Amoiba^  there  can  be 
no  doubt  that  they  consist  mainly  of  carbon  dioxide,  water,  and 
some  form  of  nitrogenous  matter  (urea  or  a  related  substance). 
Most  of  these  waste  matters  are  believed  to  be  j:)assed  out  {se- 
cretion^  excretion)  by  means  of  the  contractile  vacuole,  l)ut  prob- 
ably carbon  dioxide  leaves  the  body  by  diffusion  through  the 
general  surface  {respiration  in  part). 

The  materials  for  the  constructive  process  {anaholisin)  are 
derived  from  organic  food-matters — bodies  or  fragments  of  })lants 
and  animals  taken  as  food  in  the  process  of  alimentation^  and 
absolution  from  the  water  and  the  inorganic  salts  diss(jlved  in 
it,  and  from  the  free  oxygen  that  enters  by  diffusion  through 
the  general  surface  {respiration  in  part).  Proteid  matter  is  an 
indispensable  constituent  of  the  food,  and  Amaiba  is  therefore 
an  animal. 

Alimentation,  absorption,  secretion,  digestion,  and  circula- 
tion, all  of  which  are  only  the  prelude  to  metabolism,  but  which 
in  the  higher  animals  are  assigned  to  different  organs,  tissues, 
and  cells,  are  here  performed  by  one  and  the  same  cell.  The 
capture  of  solid  food  here  requires  its  entrance  into  the  cell ; 
and  the  fact  that  proteids  cannot  be  absorbed  by  diffusion  neces- 
sitates intracellular  digestion  which  in  turn  necessitates  cellular 
defaecation.  It  will  be  observed  that  while  there  is  no  localized 
or  permanent  mouth  or  anus,  the  whole  surface  of  the  cell  is 
potentially  mouth  or  anus.  In  short,  the  protoplasm  here  ex- 
hibits not  the  physiological  division  of  labor,  but  its  absence. 

(4)  Growth  and  Reproduction.  Logically  there  is  in  the 
case  of  Amoeba  no  good  ground  for  a  distinction  between  these 
processes  and  metabolism ;  for  reproduction  is  directly  or  indi- 
rectly an  effect  of  growth,  and  growth  is  simply  an  excess  of 
anabolism  over  katabolism.  Practically,  however,  the  distinc- 
tion is  necessary;  for  the  tendency  of  living  things  to  run  in 
cycles  of  growth  and  reproduction  is  one  of  their  most  obvious 
and  characteristic  features. 

Here,  as  in  all  protoplasmic  structures,  growth  takes  })lace 
throughout  the  mass,  by  intussusception  (p.  4),  not  by  the  ad- 


166  UNICELLULAR  ANIMALS. 

ditions  of  superficial  layers,  as  in  the  case  with  growth  by  accre- 
tion (inorganic  bodies,  e.g.,  crystals).  Under  favorable  condi- 
tion of  nutrition  this  process  exceeds  the  destructive  process  so 
that  the  body  increases  in  size  up  to  a  limit,  at  which  fission 
takes  place.  AVhat  determines  this  limit  is  unknown,  but  the 
cause  is  j)erhaps  in  some  way  connected  with  the  geometrical 
principle  that  the  volume  of  the  cell  increases  as  the  cube  of  its 
diameter,  whereas  the  surface,  by  wliich  it  absorbs  nutriment, 
and  otherwise  comes  into  relation  with  the  outside  world,  in- 
creases only  as  the  square  of  the  diameter.  'No  great  increase 
in  size,  therefore,  is  possible  without  destroying  the  normal  equi- 
librium of  the  cell  and  hence  the  periodic  reduction  of  size  by 
division.  This  principle  is,  however,  too  general  to  be  of  much 
value.  Different  species  of  Amceha  differ  in  size-limit,  and  the 
immediate  cause  lies  in  some  subtle  relation  between  organism 
and  environment  that  cannot  at  present  be  made  out.  It  is  not 
known  whether  or  not  the  Amoeba  ever  dies  of  old  age. 

These  "fundamental  physiological  properties"  of  proto- 
plasm lie  at  the  basis  of  all  physiology,  and  will  be  found  ap- 
plicable to  all  forms  of  life  whether  vegetal  or  animal. 

Related  Forms.  Amoeba  is  a  representative  of  a  very  extensive  class  of 
Protozoa  known  as  Rhizojjoda^  all  characterized  by  the  power  to  form 
pseudopodia,  and  agreeing  with  Amoeba  in  many  other  respects.  One  of 
the  commonest  fresh- w^ater  forms  is  the  genus  A7'ceUa  (Fig.  86,  C),  which 
even  in  the  active  phase  is  surrounded  by  a  brown  horny  membrane 
("shell")  perforated  by  a  large  rounded  opening  through  w^hich  pseudo- 
podia are  protruded.  DiMugia  (Fig.  86,  B),  also  a  common  fresh-water 
form,  builds  about  itself  a  beautiful  vase-shaped  or  retort-shaped  shell 
composed  of  sand-grains,  or  even,  in  some  cases,  of  diatom-shells.  In 
ActinopJirys^  or  the  "sun-animalcule"  (Fig.  86,  A),  the  pseudopodia  are 
stiff  needle-shaped  processes  radiating  in  every  direction. 

Among  the  marine  forms  two  groups  (orders)  are  of  especial  interest 
and  importance  ;  viz.,  the  Foraminifera,  which  secrete  a  calcareous  shell 
perforated  by  numerous  pores,  and  the  Radiolaria^  which  have  a  siliceous 
shell.  Many  of  these  forms  float  at  the  surface  of  the  water,  and  their 
cast-off  shells  have  in  former  times  accumulated  at  the  bottom  in  such 
enormous  quantities  as  to  form  beds  of  chalk  in  the  case  of  Foraminifera, 
while  the  remains  of  Radiolaria  have  made  important  contributions  to  the 
formation  of  siliceous  rocks. 


FRESU-WATER  RUIZOPODS. 


167 


\ 


\ 


V 


€^ 


f» 


^¥ 


1%I 


Fig.  86.— Group  of  cortmon  fresh-water  Rhizopods  (after  Loidy).  A,  Actinnjihriix 
sol,  the  "sun-animalcule,"  filled  with  vacuoles  and  containin.LT  three  food-bodies 
(zoospores  of  an  alga) ;  a  fourth  is  just  being  ingulfed.    The  nucleus  is  i^ot  seen. 

B,  Difflugia  urceolata,  with  shell  built  of  sand-grains  and  pseudopodia  far  ex- 
tended. 

C,  Arcella  mitrata,  a  transparent  individual  showing  the  protoplasmic  body  sus- 
pended within  the  shell ;  several  vacuoles  are  shown,  but  no  nucleus. 

(Highly  magnified.) 


CHAPTEE  XIII. 

UNICELLULAR   ANIMALS  (PROTOZOA)   {Continued). 

B.    Infusoria. 

{Paramoecium,  Vorticella,  etc.) 

Infusoria  are  minute  unicellular  animals  found  like  Amoeba 
in  stagnant  water  or  in  organic  infusions  (see  p.  201)  (hence 
"Infusoria").  In  the  leading  features  of  their  organization 
they  are  closely  similar  to  Amoeba  and  its  allies,  from  which 
they  differ,  however,  in  having  a  much  higher  degree  of  differ- 
entiation, in  mo^dng  by  means  of  cilia  instead  of  j)seudopodia, 
and  in  showing  the  first  indication  of  gamogenesis  (amphimixis). 
Paramoeciimi  (the  slipper-animalcule)  is  an  actively  free- 
swimming  form  often  found  in  multitudes  in  hay-infusion  or 
water  containing  the  decomposing  remains  of  Nitella  and  other 
water-plants.  Yorticella  (the  "bell-animalcule  ")  is  commonly 
attached  by  a  slender  stalk  to  duck-weed  (Lemnoi)  and  other 
water-plants,  or  to  other  submerged  objects ;  at  other  times  it 
breaks  loose  from  the  stalk  and  swims  for  a  while  actively  about. 
The  two  forms  are  constructed  upon  essentially  the  same  plan, 
but  Yorticella  shows  in  some  respects  a  much  higher  degree  of 
differentiation. 

Paramoecium. — The  slipper-shaped  body  (Fig.  87)  is  covered 
with  cilia  by  means  of  which  the  animal  rapidly  swims  about. 
Morphologically  the  body  is  a  single  cell,  having  the  same  gen- 
eral composition  as  in  Amoeba^  but  possessing  in  addition  a  deli- 
cate surrounding  membrane  ("cuticle")  or  cell-wall.  The 
differentiation  of  the  protoplasm  into  ectoplasm  and  entoplasm 
is  very  sharply  marked,  and  the  former  contains  numerous 
peculiar  rod-like  bodies  {trichoGysts)  from  which  long  threads  may 
be  thrown  out.  Their  function  is  probably  that  of  offence  and 
protection.     As  in  Amoeba  the  protoplasm  contains  water-vacu- 

oles  (w.v)  and  food-vacuoles  (f.v)   (both  of  which  are  carried 

168 


TUE  SLIPPER-ANIMALCULE. 


169 


?7^tO 


cv* 


-mm^^-  ■ " 


Fig.  87.— Param(xcium  caudatum.  A,  from  the  left  side,  showing:  the  anal  spot :  7?, 
from  the  ventral  side,  showing  the  vestibule  en  f(xcc;  arrows  inside  the  body  in- 
dicate the  direction  of  protoplasmic  currents,  those  outside  the  direction  of 
water-currents  caused  by  the  cilia. 
an,  anal  spot;  c.r,  contractile  vacuoles;  /c,  food-vacuoles ;  ic.r,  water  v;icuoles;  m. 
mouth;  mnc,  macronucleus;  »?iiV',  micronucleus;  tt',  oesophagus :  i',  vestibule.  Thi> 
anterior  end  is  directed  upwards. 


170  UNICELLULAR  ANIMALS. 

about  by  currents  in  the  entoplasm),  and  two  very  large  contrac- 
tile  vacuoles  (c.v)  occupying  a  constant  position,  one  near  either 
end  of  the  body.  The  nucleus  (as  in  Infusoria  generally)  is 
differentiated  into  two  distinct  parts,  viz. ,  a  large  oval  macro- 
nucleus  {mac.)  and  a  much  smaller  spherical  micromccleus  (mic.) 
(double  in  some  species)  lying  close  beside  it. 

Unlike  Aramha^  ParamrKjecium  possesses  a  distinct  mouth  (m) 
and  oesophagus  {<£)  which  open  to  the  exterior  through  an  oblique 
funnel-shaped  depression  known  as  the  vestibule  (v)  situated  at 
one  side  of  the  body.  Minute  floating  food-particles  are  drawn 
by  the  cilia  into  the  mouth  and  accumulate  in  a  ciliary  vortex  at 
the  bottom  of  the  oesophagus.  From  time  to  time  a  bolus  or 
food -mass  is  thence  passed  bodily  into  the  substance  of  the  en- 
toplasm, forming  a  food -vacuole  within  w^hich  digestion  takes 
place.  The  indigestible  remnants  are  Anally  passed  out  not 
through  a  permanent  opening  or  anus,  but  by  breaking  through 
the  protoplasm  at  a  definite  point,  hence  known  as  the  anal 
spot,  which  is  situated  near  the  hinder  end  (Fig.  87).  The 
contractile  vacuoles  of  Paramoecium  are  especially  favorable  for 
study,  showing  at  the  moment  of  contraction,  or  just  before  it, 
a  pronounced  star-shape,  with  long  canals  running  out  into  the 
protoplasm.  Through  these  liquid  is  supposed  to  flow  into  the 
vacuole. 

Like  Amoeba,  Paramoecium  occurs  both  in  an  active  and  in 
an  encysted  state.  In  the  former  state  it  multiplies  by  trans- 
verse fission,  division  of  both  macronucleus  and  m^icronucleus 
preceding  or  accompanying  that  of  the  protoplasmic  body  (Fig. 
88,  A).  Under  favorable  conditions  division  may  take  place  once 
in  twenty-four  hours,  or  even  oftener.  This  process,  which  is  a 
typical  case  of  agamogenesis,  may  be  repeated  again  and  again 
throughout  a  long  period.  But  it  appears  from  the  celebrated 
researches  of  Maupas  that  even  under  the  most  favorable  con- 
ditions of  food  and  temperature  the  process  has  a  limit  (in  the 
case  of  Stylonichia,  a  form  related  to  Paramoeciuim,  this  limit 
is  reached  after  about  300  successive  fissions).  As  this  lunit  is 
approached  the  animals  become  dwarfed,  show  various  signs  of 
degeneracy,  and  finally  become  incapable  of  taking  food.  The 
l^ace  grows  old  and  dies. 

In  nature,  however,  this  limit  is  probably  seldom  if  ever 


CONJUGATION  OF  PAliAM(ECIUM. 


171 


readied,  and  tlie  degenerative  tendency  seems  to  be  checked  by 
a  process  known  as  conjugation.  In  this  j^rocess  two  individuals 
place  themselves  side  by  side,  partially  fuse  together,  and  remain 
thus  united  for  several  hours  (Figs.  SS,  B,  C).  During  this 
union  an  exchange  of  nuclear  material  is  effected,  after  which 
the  annuals  separate,  both  macro7tucleus  and  micronucleus  now 


cir 


cir 


A  B 

Fig.  88.—^.  Fission  of  Paramoecium.    (From  a  preparation  by  G.  N.  Calkins),    mac, 
macronucleus  ;  mic,  micronucleus  ;  m,  mouth. 

B.  First  stage  of  conjugation.  The  animals  are  applied  by  their  ventral  sur- 
faces; the  only  change  thus  far  is  the  enlargement  of  the  micronuclei. 

C.  Conjugation  at  the  moment  of  exchange  of  the  micronuclei  (less  magnified). 
The  macronuclei  are  degenerating.  Each  individual  contains  two  micronuclei 
(now  spindle-shaped),  one  of  which  remains  in  the  body,  while  the  other  crosses 
over  to  fuse  with  the  fixed  micronucleus  of  the  other  individual  (After  Maupas.) 

consisting  of  mixed  material  derived  equally  from  both  individ- 
uals. Separation  of  the  two  animals  is  quickly  followed  by 
fission  in  each. 

In  each  individual  the  macronucleus  breaks  up  and  disappears.  The 
micronucleus  of  each  divides  twice,  and  of  the  four  bodies  thus  produced 
three  disappear.  The  fourth  divides  again  into  two,  one  of  which  remains 
in  the  body,  while  the  other  crosses  over  and  fuses  with  one  of  the  micro 
nuclei  of  the  other  individual,  after  which  the  animals  separate.  This 
process  being  reciprocal,  each  individual  now  contains  a  micronucleus  con- 


172 


UNICELLULAR  ANIMALS. 


taining  an  equal  amount  of  material  from  each  individual.  This  micro- 
nucleus  now  divides  twice  and  gives  rise  to  four  bodies,  two  of  which  be- 
come macronuclei  and  two  micronuclei.  Fission  next  occurs,  and  is  there- 
after continued  in  the  usual  manner. 

This  is  a  process  clearly  analogous  to  the  union  of  the  germ- 
cells  of  higher  animals.  It  cannot,  however,  be  called  gamo- 
genesis  or  even  reproduction ;  it  is  only  comparable  with  one  of 
the  elements  of  gamogenesis.     In  the  metazoon  a  fusion  of  two 


Fig.  89.— Group  of  Vorticellce,  in  various  attitudes,  attached  to  the  surface  of  a 

water-plant. 

cells  (fertilization)  is  followed  by  a  long  series  of  cell-divisions 
(cleavage  of  the  ovum),  the  resulting  cells  being  associated  to 
form  one  new  individual.  In  the  Infusoria  temporary  fusion 
(conjugation)  is  likewise  followed  by  a  series  of  cell-divisions, 
but  the  cells  become  entirely  separate,  each  being  an  individual. 
Vorticella  agrees  with  Paj^mnmcium  in  general  structure,  but 
differs  in  many  interesting  details,  most  of  which  are  the  expres- 


THE  BELL-ANIMALCULE. 


173 


mac 


Fig.  90.— a  single  head  of  Vorticdla.,  highly  magnified.  c.r,  contractile  axis  of  the 
stalk;  c,  cuticle;  c.i\  contractile  vacuole;  d,  disk;  fc,  ectoplasm;  t/j,  entoplasm; 
ep,  epistome  ; /.i%  f  ood-vacuole ;  m,  mouth;  »mc,  macronucleus  ;  mic,  micronu- 
cleus  ;  0?,  oesophagus;  p,  peristome ;  r,  vestibule ;  ir.r,  water-vacuole-s ;  x\  point  at 
which  epistome  and  peristome  meet  at  one  end  of  the  vestibule. 


174  UNICELLULAR  ANIMALS. 

sion  of  higher  differentiation.  The  body  is  pear-shaped  or  coni- 
cal, attached  at  its  apex  by  a  long  slender  stalk.  The  latter 
consists  of  a  slender  contractile  axial  filatnent^  by  means  of 
which  the  stalk  may  be  thrown  into  a  spiral  and  the  body  drawn 
down,  and  an  elastic  sheath  (continuous  with  the  general  cuticle) 
by  which  the  stalk  is  straightened  (Fig.  90).  The  cilia  are  con- 
fined to  a  thickened  rim,  the  peristorne  (^:>),  surrounding  the 
base  of  the  cone,  which  may  be  termed  the  dislc.  At  one  side 
the  disk  is  raised,  forming  a  projecting  angle  covered  with  cilia, 
and  known  as  the  epistome  (ej)).  At  the  same  side  the  peristome 
dips  downw«ards,  leaving  a  space  between  it  and  the  ej)istome. 
This  space  is  the  vestibule  (v),  and  into  it  the  mouth  opens.  In 
it  likewise  is  situated  an  anal  spot  like  that  of  I^aramcechmi. 
The  cilia  produce  a  powerful  vortex  centering  in  the  mouth,  by 
means  of  which  food  is  secured.  The  macronucleus  {inac)  is 
long,  slender,  and  horseshoe-shaped ;  the  small  spherical  micro- 
nucleus  (mic)  lies  near  its  middle  portion.  There  is  usually 
but  one  contractile  vacuole. 

Yorticella  multiplies  by  fission,  division  of  the  protoplasm 
being  accompanied  by  that  of  the  macronucleus  and  micronu- 
cleus  (Fig.  91).  The  plane  of  fission  is  vertical  (thus  dividing 
the  peristome  into  halves),  but  extends  only  through  the  main 
body,  leaving  the  stalk  undivided.  At  the  close  of  the  process, 
therefore,  the  stalk  bears  two  heads.  One  of  these  remains 
attached  to  the  original  stalk,  while  the  other  folds  in  its  peri- 
stome, acquires  a  second  belt  of  cilia  around  its  middle  (Fig.  91), 
breaks  loose  from  the  stem,  and  s\dms  actively  about  as  the  so- 
called  "  motile  form."  Ultimately  it  attaches  itself  by  the  base, 
loses  its  second  belt  of  cilia,  develops  a  stalk,  and  assumes  the 
ordinary  form.  By  this  process  dispersal  of  the  species  is  en- 
sured. Under  unfavorable  conditions  similar  motile  forms  are 
often  produced  wdtliout  previous  fission,  the  head  simply  acquir- 
ing a  second  belt  of  cilia,  dropping  off,  and  swimming  away  to 
seek  more  favorable  surroundings.  Yorticella  may  become  en- 
cysted, losing  its  peristome  and  mouth,  becoming  rounded  in 
form,  acquiring  a  thick  membrane,  and  having  no  stalk.  In 
this  state  it  is  said  sometimes  to  multiply  by  endogenous  dimsion^ 
breaking  up  into  a  considerable  number  of  minute  rounded 
bodies    (spores)    each    of   which   contains   a   fragment    of   the 


CONJUGATION  OF    VORTICELLA. 


175 


nucleus.  These  are  "finally  liberated  by  the  bursting  of  the 
membrane,  acquire  a  ciliated  belt,  and  after  swiuuning  for  a 
time  become  attached,  lose  the  ciliated  belt,  and  develop  a  stalk 
and  peristome. 

Yorticella  goes  through  a  process  of  conjugation  which  lias 
some  interesting  peculiarities.  (1)  Conjugation  always  takes 
place  between  a  large  attached  individual  (the  macrogamete)  and 
a  much    smaller   free-swimming   individual    (the   nvicrogainete) 


c.v 


mtc 


Fig.  91.— Fission  and  conjugation  of  VorticcWa.    A.  Early  stage  of  fission,  showing- 
division  of  micronucleus  {mic)  and  macronucleus  {mac) ;  p,  peristome.     (After 
Biitschli. ) 
B,  C,  -D.    Successive  stages  of  fission  ;  in  J3  and  C  the  nuclei  have  completely  di- 
vided and  fission  of  the  cell-body  is  in  progress;  c.r,  contractile  vacuoles.    In 
JD  fission  is  complete;  the  right-hand  individual  has   acquired  a  belt  of  loco- 
motor cilia  at  r,  and  is  ready  to  swim  away. 
JE.  Conjugation  of  a  fixed  macrogamete  {ma)  with  a  free-swimming  microgamete 
{imX\\  p,  peristome,  cp,  epistome.    (After  Greeflf.) 

(Fig.  91,  K).  The  microgamete  is  formed  either  by  the  unequal 
fission  of  an  ordinary  individual,  the  smaller  moiety  being  set 
free,  or  by  tw'O  or  more  rapidly  succeeding  fissions  of  an  ordinary 
individual.  (2)  Conjugation  is  permanent  and  complete,  the 
body  of  the  microgamete  being  wholly  absorbed  into  that  of  the 


176  UNICELLULAR  ANIMALS. 

macrogamete.  Witliin  the  body  of  the  latter,  after  complicated 
changes,  the  nuclei  fuse  together,  and  this  is  followed  by  fission. 
The  analogy  of  conjugation  to  the  fertilization  of  the  Qgg  is  here 
complete.  The  conjugating  cells  show  a  sexual  differentiation, 
one  being  like  the  ovum,  large  and  fixed,  the  other  like  the 
spermatozoon,  small  and  motile. 

As  in  Paramoscium  the  macronuclei  entirely  disappear,  fusion  takes 
place  between  derivatives  of  the  micronuclei,  and  from  the  resulting  body 
both  macronuclei  and  micronuclei  are  derived. 

Euglena  and  Other  Simpler  Infusoria.  Besides  forms  like 
Parammciiim  and  Yorticella  which  bear  numerous  cilia,  there 
are  many  Infusoria  which  possess  only  one  large  lash  orflagellum. 
Of  these  Euglena^  which  is  sometimes  found  in  stagnant  water, 
sewage-polluted  pools,  etc.,  is  one  of  the  most  interesting,  inas- 
much as  it  contains  chlorophyll,  possesses  an  "  eye-spot  "  of  red 
pigment,  and  under  certain  conditions  exliibits  amoebiform 
movements. 

Compound  or  "  Colonial ''  Forms.  In  a  number  of  forms, 
closely  related  to  Yorticella,  the  individuals  ("  zooids  ")  formed 
by  fission  do  not  immediately  separate,  but  remain  for  a  time 
united  to  form  a  ' '  colony  ' '  which  may  contain  hundreds  of 
zooids.  Zoothammo7i,  a  common  species,  thus  forms  a  beautiful 
tree-like  organism,  consisting  of  a  single  central  stalk  with  nu- 
merous branching  offshoots  from  its  summit,  each  twig  terminat- 
ing in  a  zooid.  The  entire  system  of  branches  is  traversed  by  a 
continuous  contractile  axis.  Carchesiurrh  is  similar,  but  the  axis 
is  interrupted  at  the  beginning  of  each  branch.  In  Epistylis 
the  entire  axis  is  non-contractile. 

Such  colonial  forms  are  of  high  interest  as  indicating  the 
manner  in  which  true  multicellular  forms  may  have  arisen. 
From  the  latter,  however,  they  differ  not  only  in  the  fact  that 
the  association  of  the  cells  is  not  permanent,  but  in  the  absence 
of  any  division  of  labor  among  the  units. 

Physiology.  Most  Infusoria  are  true  animals,  agreeing  with 
Ainoeba  in  the  essential  features  of  their  nutrition,  and  having 
the  power  to  digest  not  only  proteids,  but  also  carbohydrates  and 
fats.  ParmnoeciuTn  and  Yorticella  are  herbivorous  forms, 
feeding  upon  minute  plants,  and  especially  upon  the  bacteria- 


CnLOllOPUYLL-CONTAINING  INFUSOPxIA.  177 

Other  forms  are  omnivorous  (e.g.,  Stentor^  Bursarla)^  feeding 
both  on  vegetable  and  on  animal  food.  Others  still  are  car- 
nivorous and  lead  a  predatory  life,  often  attacking  herljivorous 
forms  much  larger  than  themselves,  precisely  as  is  the  case  with 
carnivores  among  the  mammalia.  Thus  the  unicellular  \v()rl<l 
reproduces  in  miniature  the  essential  biological  relations  of 
higher  types. 

It  is  a  remarkable  fact  that  some  species  of  Infusoria  (e.g., 
Parainoacitiin  hursaria^  Yorticella  viridis)  contain  numerous 
chlorophyll-bodies  embedded  in  the  entoj^lasm.  Much  discus- 
sion has  arisen  as  to  whether  these  bodies  are  to  be  regarded  as 
an  integral  part  of  the  animal,  i.e.,  differentiated  out  of  its  own 
protoplasm,  or  as  minute  plants  living  ' '  symbiotically  "  (i.e.  as 
mess-mates)  within  the  animal.  In  the  forme:*  case  (which  is 
the  most  probable)  the  animal  w^ould  to  a  certain  extent  be 
nourished  after  the  fashion  of  a  green  plant  (cf.  p.  lis). 

It  will  now  be  clear  to  any  one  wdio  has  carefully  considered 
the  phenomena  described  in  the  foregoing  pages  that  the  uni- 
cellular animals  are  "organisms"  by  right,  and  not  merely  by 
courtesy.  In  some  of  the  Infusoria,  for  example,  differentia- 
tion within  the  single  cell  may  go  so  far  as  to  give  rise  to  primi- 
tive sense-organs  (as  in  the  case  of  the  eye-spot  of  Euglena) ;  a 
rudimentary  oesophagus  and  definite  mouth  (as  in  Parammciiuri 
and  Yorticella) ;  organs  of  locomotion  (cilia,  flagelld) ;  organs 
of  excretion  (contractile  vacuoles)  etc. ,  etc. 


CHAPTEE  XIY. 

UNICELLULAR  PLANTS. 

A.  Protococcus. 

{Protococcus,  Pleurococcus,  Chlorococcus,  Hcematococcus,  etc.) 

Unicellular  plants,  like  unicellular  animals,  are  very  com- 
mon, although  as  individuals  mostly  invisible  on  account  of  their 
microscopic  size.  In  the  mass,  however,  they  are  often  visible 
either  as  suspended  or  floating  matter,  causing  ' '  turbidity  ' '  in 
liquids  (yeast,  bacteria,  diatoms,  desinids,  etc.)  or  discolorations 
on  tree-trunks,  earth,  stones,  roofs,  and  flower-pots.  {Pro- 
tococcus, Glmocapsa,  etc.). 

Under  the  term  Protococcus  {jiporo^,  first,  kokko^,  herry) 
we  may  for  our  present  purposes  include  a  number  of  the  simplest 
spherical  forms,  generally  green  in  color  and  of  uncertain  afiin- 
ities  in  classifl cation,  but  very  sunilar  in  structure,  living  for  the 
most  part  in  quiet  waters  or  on  moist  earth,  stones,  tree-trunks, 
or  old  roofs,  or  in  water-butts,  roof-gutters,  and  the  like. 
Sometimes  the  color  which  they  exhibit  is  yellowish-green, 
sometimes  bluish-green,  and  sometimes,  though  less  often,  reddish, 
according  to  the  species. 

One  of  the  commonest  and  most  conspicuous  is  a  species 
often  seen  on  the  shady  side  of  old  tree-trunks  where,  when 
abundant,  it  forms  a  greenish  dust-like  coating  or  discoloration, 
scarcely  visible  when  dry  but  becoming  a  rich  bright  green  dur- 
ing prolonged  rains  or  after  warm  showers.  If  pieces  of  bark 
covered  with  this  form  of  Protococcus  are  moistened,  the  green- 
ish coating  may  be  observed  at  any  time.  It  is  granular  in  tex- 
ture and  after  moistening  is  easily  loosened  by  a  camePs-hair 
brush. 

Morphology.  Microscopical  examination  shows  that  the  par- 
ticles detached  consist  of  rounded  yellowish-green  cells  occurring 
either  singly  or  in  groups  of  two,   three,  four,  or  even  more. 

178 


PROTOCOCCUS,  179 

Each  single  cell  is  a  complete  individual,  capable  of  carrying  on 
an  independent  life.  It  fairly  represents  the  green  plant  (such 
as  Pteris)  reduced  to  its  lowest  terms.     (Fig.  92.) 

Like  Amoeha  and  the  Infusoria  Protococcus^  at  least  in  some 
species,  occurs  both  in  a  motile  or  active  state  in  which  it  moves 
about,  and  a  quiescent  or  non-inotile  state  analogous  to  the  en- 
cysted state  of  the  unicellular  animals.  In  the  latter  the  motile 
or  active  state  is  the  usual  or  dominant  condition  and  tlie  en- 
cysted state  is  rarely  assumed.  In  Protococcus^  on  the  other 
hand,  the  motile  state  is  rare,  and  the  ordinary  activities  of  tlie 
plant  are  carried  on  in  the  non-motile  state. 

Structure.  In  structure  Protococcus  is  a  nearly  typical  cell 
(p.  22).  It  consists  essentially  of  an  approximately  spherical 
mass  of  protoplasm  enclosed  within  a  thin  woody  layer  of  cellu- 
lose (cell-wall  or  cell-membrane),  and  contains  a  single  nucleus. 
It  also  includes  one  or  more  chlorojphyll-hodies  (chromatophores) 
(p.  126)  by  virtue  of  which  it  is  able  to  manufacture  its  own 
foods,  very  much  after  the  fashion  of  the  green  cells  of  Pteris. 

In  those  forms  which  possess  a  motile  stage  the  latter  con- 
sists of  a  spherical,  egg-shaped  or  pear-shaped  cell  having  cliro- 
matophores  and  a  membrane  through  which  two  flagella  protrude. 
In  the  oval  forms  these  are  placed  near  the  narrowed  end  of  the 
cell,  and  in  all  cases  they  are  locomotor  organs  and  propel  the 
cell  swiftly  through  the  water.     (Fig.  92). 

Reproduction.  The  ordinary  method  of  reproduction  in  the 
unicellular  plants,  as  in  the  unicellular  animals,  is  by  cell-division. 
In  Protococcus  the  sphere  becomes  divided  by  a  partition  into 
two  cells  which  eventually  separate  completely  one  from  the 
other.  Yery  often,  however,  the  separation  being  incomplete 
or  postponed  until  after  each  daughter-cell  has  in  turn  become 
divided,  groups  or  aggregates  of  cells  arise  which  suggest  the 
first  steps  in  the  formation  of  tissue  in  the  devel(>}unent  of  higher 
forms.  In  the  end,  however,  separation  is  total  and  complete, 
and  each  cell  is  therefore  not  a  unit  in  a  body,  but  is  itself  a 
body  and  an  individual  (see  p.  156).      (Fig.  92.) 

The  daughter-cells  thus  produced  are  the  young,  or  offsiiring, 
which  have  the  power  to  grow  and  ultimately  to  divide  in  their 
turn.  Under  favorable  circumstances  generation  may  thus  fol- 
low generation  in  quick  succession.      Each  young  cell  is  actually 


180 


UNICELLULAE  PLANTS. 


Fig.  93. — Protococcus  (Pleurococcus)  from  the  bark  of  an  elm  tree,  in  active  vegeta- 
tion and  showing  aggregation  into  masses  of  cells.  A,  Pleurococcus  in  the  dried 
condition.  JB,  Ascococcus  (?),  showing  endogenous  division  into  two  cells  and  (Q 
into  four.    D,  E,  F,  motile  forms  of  Protococcus  (after  Cohn). 


NUTRITION  OF  PliOTOCOCCUS.  181 

one  half  of  the  parent  cell  and  contains  a  moiety  of  whatever 
that  contained.  Here,  therefore,  as  in  Amaha^  the  problems 
of  heredity,  uncomplicated  by  the  occurrence  of  sex,  are  reduced 
to  their  lowest  terms. 

In  some  kinds  of  Protococciis  the  quiescent  cells,  under 
special  circumstances,  which  are  not  well  understood,  t^ive  rise 
to  the  motile  forms  {zoospores)  referred  to  above.  Cilia,  or 
rather  flagella,  are  formed,  and  the  protoplasmic  mass  with  its 
included  chromatophores  swims  actively  about  in  the  water. 
After  a  time  these  motile  cells  may  come  to  rest,  lose  their  fla- 
gella and  divide  into  two  or  more  daughter-cells,  each  of  which 
in  its  turn  may  become  a  motile  cell  and  repeat  the  process,  or, 
under  other  conditions,  develop  into  the  ordinary  quiescent  cell. 

In  some  species  of  Protococciis  in  which  there  is  a  motile 
stage  another  form  of  reproduction,  a  kind  of  rudimentary 
gamogenesis,  has  been  observed.  In  this  process  two  of  the 
motile  cells  (gametes)  meet,  fuse  (conjugation)^  lose  their  flagella, 
become  encysted  (see  p.  161),  and  ultimately  give  rise  to  the 
ordinary  cells  of  Protococciis^  both  non-motile  and  motile. 
This  process,  however,  has  not  yet  been  observed  in  the  species 
under  consideration. 

Physiology.  Our  actual  knowledge  of  the  physiology  of 
Protococciis  is  very  small.  But  the  study  of  comparative  plant 
physiology  gives  every  reason  to  believe  that  the  essential  phys- 
iological operations  of  this"  simple  plant  are  fundamentally  of 
the  same  character  as  in  the  higher  green  plants,  such  as  Pteris. 

Nutrition.  The  income  of  Protococciis^  when  growing  in 
its  natural  habitat  on  tree-branches,  moist  bricks,  and  the  like, 
is  difficult  to  determine.  But  as  it  is  able  to  live  also  in  ordi- 
nary rain-water,  we  are  able  to  set  down  its  probable  income 
under  those  conditions  with  some  deo^ree  of  accuraev.  There 
is  do  doubt  that  it  absorbs  water  and  carl)on  dioxide  by  dif- 
fusion through  the  cellulose  wall,  and  that  these  substances 
are  used  in  the  manufacture  of  starch,  which,  if  stored  up, 
makes  its  appearance  in  the  form  of  small  graTiulos  within  the 
chromatophores.  This  process  takes  place  only  in  the  light  and 
through  the  agency  of  the  chlorophyll,  and  is  attended  by  a 
setting  free  of  oxygen  precisely  as  in  Pteris.  Nitrogen  is  prob- 
ably derived  from  nitrates  or  ammoniacal  compounds,  minute 


182 


UNICELLULAR  PLANTS. 


quantities  of  which  are  dissolved  in  the  water,  and  other  neces- 
sary salts  (sulphates,  chlorides,  j)hosphates,  etc.)  as  well  as  free 
oxygen  are  procured  from  the  same  source.  These  substances 
may  be  derived  from  dust  blown  or  washed  by  the  rain  into  the 
water,  or  from  the  walls  of  the  vessel.  To  the  process  of  starch- 
making,  attended  by  the  absorption  of  CO^  and  HjO  and  the 
liberation  of  O,  the  term  "assimilation"  is  generally  given. 
Like  other  plants,  moreover,  Protococcus  probably  breathes 
by  absorbing  free  oxygen  and  setting  free  CO,  (respiration). 

The  income  and  outgo  of  Protococcus  may  then  be  displayed 
by  the  following  diagram : 


H20 


FreeO 


Assimilation 


Incojne^ 


Nitrates  or 

Ammonia  Comp. 

Sulphates 

and 
ctlier  Salts 


Respiration 

FreeO 


Urea  (?) 
Sulphates 

and 
other  Salts 


Outgo 


It  should  be  understood  that  this  only  represents  the  broad 
outlines  of  the  process  and  under  the  simplest  conditions.  It  is 
quite  possible  that  under  other  conditions  Protococcus  may  use 
more  complex  foods.  The  facts  remain,  however,  (1)  that 
Protococcus  is  dependent  on  the  energy  of  light ;  (2)  that  its 
action  is  on  the  whole  constructive,  resulting  in  the  formation  of 
complex  compounds  (carbohydrates,  proteids)  out  of  simpler 
ones.  In  these  respects  it  shows  a  complete  contrast  to  Amosha, 
which  is  on  the  whole  destructive,  breaking  down  complex  com- 
pounds into  simpler  ones,  and  is  indej)endent  of  light,  since  it 
derives  energy  from  the  potential  energy  of  its  food.  The 
relations  between  Protococcus  and  Amoeha  are  therefore  an 
epitome  of  the  relations  between  Pteris  and  Luiiibricus^  and 
between  green  plants  and  animals  generally. 

The  Fundamental  Physiological  Properties  of  Plants.  In  con- 
sidering the  physiology  of  Amoeba  we  found  it  possible  to  re- 


PROTOCOCCUS  AND  AMGEBA    COMPARED.  183 

t 

duce  its  vital  activities  to  a  few  fundamental  pliysi(jl(jgical  proper- 
ties, namely,  contractility,  irritability,  metaljijlism,  grcjwtli  and 
reproduction,  common  to  all  animals.  A  little  rejection  will 
show  that  the  same  properties  are  manifested  also  by  Pmtn- 
eoccus.  Contraction  and  irritability  are  difficult  to  witness  iu 
the  quiescent  stage  of  Protococcus^  but  obvious  enougli  in  tlie 
rarer  motile  forms.  Metabolism,  growth  and  reproduction,  on 
the  other  hand,  are  evident  accompaniments  of  normal  life,  even 
in  the  quiescent  condition.  And  precisely  as  Protococcus  diifers 
from  Amaiba  in  respect  to  contractility  and  irritability,  of  wliicli 
it  possesses  relatively  little,  so  plants  in  general  differ  in  tliese 
respects  from  animals  in  general.  Animals  are  eminently  con- 
tractile and  irritable,  while  plants  are  but  feebly  specialized  in 
these  directions.  On  the  other  hand,  as  we  have  already  seen 
in  comparing  Pteris  with  Lumhricus  (p.  154),  and  as  we  see 
once  more  in  comj)aring  Protococcus  with  Amoeba^  in  respect  to 
metabolism,  the  green  plant  is  pre-eminently  constructive,  while 
the  animal  is  preeminently  destructive,  of  organic  matter. 

In  their  modes  of  nutrition,  as  stated  above.  Amoeba, 
and  Protococcus  represent  two  physiological  extremes.  AVe 
pass  now  to  the  study  of  Yeasts  and  Bacteria,  which  are  plants 
destiUite  of  chlorophyll  and  in  a  certain  sense  may  be  regarded 
as  occupying  a  middle  ground  between  these  extremes. 

Other  Forms.  There  are  innumerable  species  of  uniceUular  green 
plants.  A  vast  group  of  peculiar  brownish  forms  covered  with  transparent 
^lass-like  cells  composed  of  siliceous  material  is  known  as  the  Diato- 
macecE  or  diatoms.  In  these  the  chlorophyll  is  masked  by  a  brown  pig- 
ment, but  is  nevertheless  present.  Another  group  is  that  known  as  the 
Desmidice  or  desmids.  These  often  have  the  individual  cells  peculiarly 
constricted  in  the  middle  so  that  at  first  sight  the  two  halves  appear  to  be 
two  separate  cells.  More  closely  resembling  Protococcus  in  many  respects 
are  some  members  of  the  Cyanophycew  or  "blue-green  algiv,"  among 
which  Ghro'ococcus  and  Glceocapsa  differ  from  Protococcus  chiefly,  in  the 
former  case,  in  having  a  blue-green  instead  of  a  yellow-green  pigment, 
and,  in  the  latter,  not  only  in  this  respect,  but  also  in  the  fact  that  the 
single  cells  are  widely  separated  by  transparent  mucilage. 


CHAPTEE  XV. 


UNICELLULAR  PLANTS  {Continued). 

B.  Yeast. 

{Saccharomyces.) 

Under  tlie  general  name  of  yeast  are  included  some  of  the 
simplest  forms  of  vegetal  life.  Some  yeasts  are  ''wild,"  liv- 
ing upon  fermenting  fruits  or  in  fruit  juices,  and  commonly 


Fig.  93.— Yeast-cells.  Brewer's  (top)  yeast  actively  vegetating.  The  large  internal 
vacuoles  and  the  small  fat-drops  are  shown,  as  are  also  buds,  in  various  stages  of 
development,  and  the  cell-wall.    Nuclei  not  visible.    (Highly  magnified.) 

oecurring  in  the  air ;   others  are  ' '  domesticated, ' '  or  cultivated, 
such  as  those  regularly  employed  in  bremng  and  in  baking. 

If  a  bit  of  "yeast-cake  "  (either  "comj^ressed  "or  "  dried" 
yeast)  is   mixed  with  water,  a  milky  fluid  is   obtained  which 

closely  resembles  the  so-called  baker's  or  brewer's  yeast. 

184 


STRUCTURE  OF  YEAST. 


185 


Microscopical  examination  proves  that  the  milky  appearance 
of  liquid  yeasts  is  due  chieliy  to  the  presence  of  myriads  of 
minute  egg-shaped  suspended  bodies,  and  tliat  pressed  yeast  is 
almost  wholly  a  mass  of  similar  forms.  These  are  the  cells  of 
yeast ;  which  is  therefore  essentially  a  mass  of  unicellular  organ- 
isms.    For  reasons  which  will  soon  appear  yeast  is  universally 


Fig.  94.— Yeast-ceUs.    Brewer's  (bottom)  yeast  showing  structure— protoplasm,  cell- 
walls,  vacuoles,  fat-drops.    (Nuclei  not  shown.) 

regarded  as  a  plant,  and  the  single  cell  is  often  sj^oken  of  as  the 
yeast -plant. 

Morphology.  The  particular  yeasts  which  we  shall  consider  are 
the  common  cultivated  forms  of  com- 
merce. The  cells  of  an  ordinary  cake 
of  pressed  yeast  are  spherical,  sphe- 
roidal, or  egg-shaped  in  form,  and  con- 
sist of  a  mass  of  protoplasm  enclosed 
within  a  well-deiined  cell-wall.  Bv 
ap2)ropriate  treatment  the  latter  may 
be  shown  to  consist  of  cellulose ;  and 
it  is  distinctly  thicker  in  old  or  resting  Fm.  n5.-Spore9  of  Yeast  (As- 

^  'J  ^  °      cospores).  Four  spores  in  a  cell 

cells    than  in  young  ones  or  those  vig-      of  brewer's  yeast  (^atc/KiromyftViJ 

orously  growing.    Within  the  granular    «<^'''*^'''"""')' 
protoplasm  {cyto2)lasm)  are  usually  a  number  of  vacuoles  (con- 
taining sap)  and  minute  shining  dots  (probably  fat-droplets\  but 


186 


UNICELLULAR  PLANTS. 


no  chlorophyll  is  present  and  no  starch.  Until  recently  the  yeast- 
cell  was  snj^posed  to  be  destitute  of  a  nncleiis,  but  it  isnow  kno\Mi 
that  each  cell  probably  jDossesses  a  large  and  characteristic  nuclens. 
This,  however,  can  be  demonstrated  only  by  s]3ecial  reagents  and 
is  rarely  or  never  seen  in  the  living  cell  (Fig.  96). 

Reproduction.      The  ordinary  mode  of  reproduction  of  yeast 
is  by  a  modification  of   cell-division    called    hudding.      Under 


Fig,  96.— The  Nuclei  of  Yeast-cells  and  the  Process  of  Budding.  (Drawn  by  J.  H. 
Emerton  from  specimens  prepared  by  S.  C.  Keith,  Jr.)  The  upper  left-hand  figure 
shows  the  nucleus  in  a  specimen  treated  with  Delafl eld's  hsematoxylin.  The 
other  figures  in  the  upper  row  and  those  in  the  lower  (from  left  to  right)  show 
cells  in  successive  stages  of  budding,  together  with  the  appearance,  position,  and 
movements  of  the  nucleus.  It  will  be  observed  that  the  bud  is  formed  before  the 
nucleus  divides.    (Iron-haematoxylin  method.) 

favorable  circumstances  in  actively  growing  yeast  a  local  bulging 
of  the  wall  takes  place,  usually  near,  but  not  precisely  at,  one 
pole  of  the  cell.  Protoplasm  presses  into  this  dilatation  or 
''  bud  "  and  extends  it  still  further.  At  this  time  we  have  still 
but  one  cell,  although  it  now  consists  of  two  unequal  parts  and 
the  separation  of  a  daughter-cell  is  clearly  foreshadowed.  Event- 
ually the  connection  between  the  two  parts  is  severed  and  the 
daughter-cell  or  ' '  bud  ' '  is  detached  from  the  original  or  j^arent- 
cell ;  but  detachment  may  or  may  not  occur  until  after  the  bud 


FORMATION  OF  SPORES  IN   YEAST. 


187 


has  begun  to  produce  dangliter-cells  in  its  turn,  and  nmi-t'  than 
one  bud  may  be  borne  by  either  or  b<jth  parent-  or  dauglitcr- 
cells.  In  very  rapid  growth  the  connection  may  persist  between 
the  cells  even  during  the  formation  of  several  generations  of 
buds;  but  this  is  unusual,  and  in  cases  where  a  number  of  cells 
remain  apparently  united  together  forming  tree-like  forms  there 
is  often  no  real  connection,  the  cells  separating  readily  on  agita- 
tion. 

Endospores  (Ascospores).      Some    yeasts   in    additicjn    to    the 
method  of  reproduction  by  budding  exhibit  another  mode  known 


Fig.  97.— Spores  of  Yeast  (Ascospores).     Three-  and  two-celled  stage  of  spore  for- 
mation in  S.  cereviske. 

as  endogenous  division  or  ascospore  formation.  Under  certain 
circumstances  not  yet  entirely  understood  there  are  formed 
within  the  yeast-cell  two,  three,  or  four  rounded  shining  spores. 
These  become  surrounded  by  thick  walls  and  thus  give  rise 
eventually  to  a  group  of  daughter-cells  within  the  original  cellu- 
lose sac.  To  the  latter  the  term  ascus  (sac)  has  been  applied, 
and  to  its  contained  daughter-cells  the  term  ascosjjores. 

It  is  not  yet  allowed  by  all  botanists  that  this  terminology,  wliich  im- 
plies a  relationship  of  yeasts  to  the  Ascoraycetous  fungi,  is  sound  ;  but  it 
is  commonly  used. 

Each  ascospore  is  capable  under  favorable  circumstances  c»f 
sprouting  and  starting  a  new  series  of  generations  of  ordinary 
yeast-cells..  It  should  be  particularly  observed  that  the  eudi.- 
spores  of  yeast  are  reproductive  bodies,  and  that  the  process  of 
their  formation  is  one  of  multiplication — not  merely  one  of  de- 
fence or  protection,  as  is  the  case  with  the  so-called  "spores  '' 
of  bacteria  described  beyond  (p.  11»4). 


188  UyiCELLULAR  PLANTS. 

Physiology.  Like  all  other  organisms  the  yeast-plant  occu- 
pies a  definite  j)osition  in  space  and  time;  it  possesses  an  en- 
vironment with  which  it  must  be  in  harmony  if  it  is  to  live, 
from  which  it  derives  an  income,  and  to  which  it  contributes  an 
outgo  of  matter  and  energy ;  it  manufactures  its  own  substance 
from  foods  {anaholism)^  and  like  all  living  things  it  wastes  by 
oxidation  of  its  substance  {kataljolism).  It  is  not  obviously  con- 
tractile or  irritable,  but  it  is  highly  metabolic  and  reproductive. 

Yeast  and  its  Environment.  Yeast  is  an  aquatic  form,  and, 
as  might  be  supposed,  cultivated  yeast  thrives  best  in  its  usual 
habitat,  the  juices  of  fruits,  such  as  apples  or  grapes,  and  the 
watery  extracts  of  sprouted  seeds,  such  as  barley,  corn,  and 
rye  (wort,  mash,  etc.).  It  lives,  however,  more  or  less  success- 
fully in  many  other  places  (such  as  the  dough  of  bread),  and  can 
even  endure  mucli  dryness,  as  is  shown  by  the  commercial 
' '  dried  -  yeast. ' '  It  appears  to  prefer  a  temperature  from 
20°  to  30°  C.  ;  it  is  usually  killed  by  boiling,  but  if  dried,  it  can 
endui'e  high  temperatures.  Its  action  is  inhibited  by  very  low 
temperatures,  but  like  most  living  things  it  endures  low  temper- 
atures better  than  high.  It  is  killed  by  many  poisons  (anti- 
septics). 

Income.  Owing  to  its  industrial  imj^ortance  yeast  has  been 
l^erhaps  more  thoroughly  studied  in  respect  to  its  nutrition  than 
any  other  unicellular  organism.  And  yet  it  is  impossible  to 
give  accurate  statistics  of  its  normal  income  and  outgo.  It  is 
believed  that  the  ordinary  income  of  a  yeast- cell  living  in  wort 
(the  watery  extract  of  sprouted  barley-grains)  consists  of  a^  dis- 
solved oxygen  j  J,  nitrogenous  Z>6>(^z^<§  allied  to  proteids,  but  diffusi- 
ble and  able  to  j)ass  through  the  cellulose  wall ;  <?,  carhohydrates, 
esj)eciaUy  sugary  matters  /  and  6?,  salts  of  various  kinds. 

It  Avas  supposed  for  a  long  time  by  Pasteur  and  others  tliat 
yeast  could  dispense  with  free  (dissolved)  oxygen  in  its  dietary. 
It  now  appears  that  this  faculty  is  temporary  only,  and  that  if 
yeast  is  to  thrive  it  must,  like  all  other  living  things,  be  sup- 
plied, at  least  occasionally,  with  free  oxygen. 

Metabolism.  Out  of  the  income  of  foods  just  described  yeast 
is  able  to  build  up  its  own  peculiar  23rotoi3lasm  {anabolism)^  and, 
doubtless,  to  lay  down  the  droplets  of  fat  which  often  appear  in 
it.     There  is  good  reason  to  believe  that  its  substance  also  breaks 


NUTRITION  OF  YEAST.  189 

down,  witli  tlie  productiiji  of  carbon  dioxide,  water,  and  nitro- 
genous waste  (kataholisM),  and  the  concomitant  liberation  of 
energy.  The  work  to  De  done  by  the  yeast-cell  is  ])]ainly 
limited.  The  manufacture  of  new  and  of  surplus  protoj)lasni 
and  the  protrusion  of  buds  require  work,  partly  chemical, 
partly  mechanical;  but  most  of  the  liberated  energy  pr(>ba])ly 
appears  as  heat.  In  point  of  fact,  great  activity  of  yeast  is 
accompanied  by  a  rise  of  temperature,  as  may  beproved  by 
placing  a  thermometer  in  "rising"  dough  or  fermenting  fruit- 
juice. 

Outgo.  Barring  the  outgo  of  energy  already  mentioned,  and 
the  probable  excretion  of  carbon  dioxide  and  nitrogenous  waste, 
but  little  can  be  said  concerning  the  outgo  of  a  yeast-cell.  The 
ordinary  excretions  are  so  masked  by  the  presence  of  foreign 
matters  in  the  liquids  which  yeast  inhabits  that  little  is  known  of 
the  real  course  of  events.  To  the  consideration  of  conditions 
which  entail  these  difficulties  we  may  now  pass,  merely  pausing 
to  caution  the  student  against  the  suj^position  that  the  evolution 
of  carbon  dioxide  in  fermentations  represents  to  any  great  ex- 
tent the  normal  respiration  of  the  yeast  cells. 

Mineral  Nutrients  of  Yeast.  It  has  been  shown  (pp.  14S,  ISl) 
that  Pteris  and  Protococcus^  inasmuch  as  they  possess  chlorophyll 
can  live  upon  simple  inorganic  matters  such  as  C0„,  II^C ),  and 
nitrates,  out  of  which  they  are  able  to  manufacture  for  them- 
selves energized  foods  such  as  starch.  Yeast  is  unal)le  to  do 
this,  as  might  be  supposed  from  the  fact  that  it  is  destitute  of 
chlorophyll.  And  yet  yeast  does  not  require  proteid  ready- 
made  as  all  true  animals  do,  for  experiments  have  shown  that  it 
can  live  and  grow  in  a  liquid  containing  only  mineral  matters 
plus  some  such  compound  of  nitrogen  as  annnonium  tartrate 
(C^H/NIIJ^Oe).  Upon  a  much  less  complex  organic  compound 
of  nitrogen  such  as  a  nitrate  it  cannot  thri\e,  thus  showing  its 
inferiority  in  constructive  power  to  Protococcus  and  all  green 
plants,  on  the  one  hand,  and  its  sui^eriority  to  Avxvha  and  all 
animals,  on  the  other. 

Pasteur's  fluid,  composed  of  water  and  salts,  amoii";  wliich  is  ammonium 
tartrate  (above),  will  suffice  to  support  yeast.  It  will  sui)port  a  much  more 
vigorous  growth  if  sugar  be  added  to  it.  But  if  ammonium  nitrate  is  sub- 
stituted for  ammonium  tartrate  yeast  will  refuse  to  grow  in  the  fluid. 


190  UNICELLULAR  PLANTS. 

Yeast  is  a  Plant.  The  superior  constructive  faculty  of  yeast, 
just  described,  separates  it  fiindamentallv  from  all  animals  in 
respect  to  its  physiology,  and  allies  it  closely  to  all  plants.  Its 
inferiority  to  the  chlorophyll-bearing  plants  or  parts  of  plants,  on 
the  otlier  hand,  in  no  wise  separates  it  fundamentally  from 
plants ;  for  it  must  not  be  forgotten  that  the  power,  even  of 
plant- cells  to  utilize  mineral  matters  as  raw  materials  and  from 
them  to  manufacture  foods  like  starch,  ordinarily  resides  exclu- 
sively  in  the  chlorophyll  bodies,  and  is  operative  only  in  the 
presence  of  light.  It  follows,  therefore,  that  most  of  the  cells, 
even  of  the  so-called  green  plants,  and  a  considerable  portion  of 
the  contents  of  the  so-called  green  cells,  must  be  destitute  of 
this  synthetic  power.  Considerations  of  this  kind  show  how 
exceedingly  localized  and  special  the  starch-making  function  is, 
even  in  the  ' '  green ' '  plants ;  and  yeast  probably  compares  very 
favorably  in  its  synthetic  powers  with  many  of  the  colorless  cells 
of  such  plants,  or  even  with  the  colorless  protoplasmic  portions 
of  chromatophore-bearing  cells. 

But  yeast  is  vegetal  rather  than  animal,  morphologically  as 
well  as  physiologically.  Its  structure  more  nearly  resembles 
that  of  some  undoubted  plants  (fungi)  than  any  animal.  Its 
wall  is  composed  of  a  variety  of  cellulose,  called  fungus-cellulose ; 
and  cellulose,  though  occasionally  occurring  in  animal  structures, 
is,  broadly  speaking,  a  vegetal  compound.  Finally,  in  its 
methods  of  reproduction  by  budding,  and  by  spores,  yeast  is 
allied  rather  to  plants  than  animals. 

Top  Yeast.  Bottom  Yeast.  In  the  process  of  brewing  two  well- 
marked  varieties  of  yeast  occur,  known  as  "  top  "  and  "  bottom  "  yeast. 
The  former  is  used  in  the  making  of  English  ale,  stout,  and  porter ;  the 
latter  in  the  making  of  German  or  "  lager  "  beer.  The  top  yeast  is  culti- 
vated at  the  ordinary  summer  temperature  of  a  room,  without  special  at- 
tention to  temperature  ;  the  latter  in  rooms  artificially  cooled  so  that  even 
in  summer,  icicles  often  hang  from  the  walls.  The  two  yeasts  also  show 
obvious  differences  in  form,  size,  and  structure  ;  and  how  much  they  must 
differ  in  their  function  is  plain  from  the  very  different  products  to  which 
they  give  rise. 

Wild  Yeasts,  Besides  the  commercial  or  cultivated  yeasts  there  are 
also  wild  yeasts,  and  to  them  are  due  in  the  main  the  fermentations  of 
apple-juice,  of  grape-juice,  and  other  fruit  juices.  A  drop  of  sweet  cider 
shows  under  the  microscope  a  good  example  of  one  of  these  species  ;  and 
Pasteur  long  ago  proved  that  the  outer  skins  of  ripe  grapes  and  other  fruits 


VARIETIES  OF   YEAST.  191 

are  apt  to  harbor  yeast-cells  in  the  dust  which  lodges  upon  them.  More 
recently  it  has  been  shown  that  wild  yeasts  often  live  under  apple-trees- 
upon  the  surface  of  the  earth.  In  a  dry  time  the  wind  easily  lifts  the  dust 
containing  them  and  conveys  them  over  great  distances  (cf.  Amwha, 
Infusoria,  etc.).  The  domesticated  yeasts  of  to-day  are  probably  the  de- 
scendants of  similar  wild  yeasts. 

Red  Yeast.  One  of  the  finest  of  the  wild  yeasts  is  the  so-called  "red 
yeast,"  which  is  furthermore  very  easy  to  study.  Red  yeast,  and  many 
others  not  red,  grow  luxuriantly  upon  a  jelly,  made  by  thickening  beer- 
wort  with  common  gelatine.  In  this  way  "pure"  cultures — that  is,  cul- 
tures free  from  other  species  of  yeasts,  or  bacteria,  and  consisting  of  one 
kind  only — can  be  easily  made  and  studied.  The  microscope  shows  that 
the  cells  of  red  yeast,  which  form  red  dots  upon  such  jelly,  are  not  them- 
selves colored,  but  the  pigment  appears  to  lie  between  the  cells,  as  in  the 
case  of  the  "  miracle  germ  "  {Bacillus  prodigiosus). 

Fermentation.  To  the  processes  where  yeast  is  employed  to- 
produce  chemical  changes  in  various  domestic,  agricultural,  and 
industrial  operations  the  term  fermentation.^  or  more  often 
alcoholic  fermentation.^  is  applied.  In  the  ''  raising"  of  bread 
or  cake,  in  brewing,  cider-making,  etc.,  yeast  acting  upon 
sugar  produces  from  it  an  abundance  of  alcohol  and  carbon 
dioxide.  Both  products  are  sought  for  in  brewing,  and  carbon 
dioxide  is  especially  desired  in  bread-making. 

But  alcoholic  fermentation  is  only  one  example  of  a  large 
class,  and  yeast  is  only  one  of  many  ferments.     We  may,  there- 
fore, postpone  further  consideration  of  fermentation  to  the  next 
chapter. 

Related  Forms.  It  has  been  shown  by  the  researches  of  Hansen  that 
ordinary  commercial  yeast  is  seldom  one  single  species,  as  was  formerly 
supposed,  but  rather  a  mixture  of  several  species.  It  is  therefore  no 
longer  safe  to  speak  of  commercial  yeast  as  Saccliaromyces  cerevisiop,  unless 
careful  examination  by  the  modern  methods  has  shown  it  to  be  such  ;  and 
to  determine  what  species  exist  in  any  particular  specimen  is  often  a  labori- 
ous and  difficult  matter. 

Inasmuch  as  the  natural  position  of  yeast  in  the  vegetal  kingdc^m  is 
not  established  beyond  all  doubt,  it  is  impossible  to  state  precisely  what 
are  its  near  relatives.  There  are  numerous  unicellular  colorless  plants,  but 
they  are  not  necessarily  closely  related  to  yeast ;  and  the  student  must  not 
conclude  for  plants  any  more  than  for  animals  that  because  an  organism 
unicellular  it  is  necessarily  at  the  very  bottom  of  the  scale  of  life. 


CHAPTER   XYI. 

UNICELLULAR  PLANTS  (Continued). 

C.  Bacteria. 

{Schizomycetes.) 

The  smallest,  and  the  most  numerous,  of  all  living  things  are 
the  bacteria.  Bacteria  occur  almost  everywhere  :  they  are  lifted 
into  the  atmosphere  as  dust  particles,  in  it  they  float  and  with  its 
currents  they  are  driven  about;  water — both  fresh  and  salt — 
often  contains  large  numbers  of  them ;  and  the  upj^er  layers  of 
the  soil  teem  with  them.  But  they  are  most  abundant  in  liquids 
containing  dissolved  organic  matters,  especially  such  as  have  stood 
for  a  time — for  example,  stale  milk  and  sewage,  these  fluids 
often  containing  millions  of  individual  bacteria  in  a  single  cubic 
centimetre. 

In  respect  to  their  abundance  in  the  surface  layers  of  the 
earth  (one  gram  of  fertile  soil  often  containing  a  million  or  more), 
and  the  work  which  they  do  there  in  producing  the  oxidation  of 
organic  matters  and  changes  in  the  composition  of  the  soil,  bac- 
teria may  well  be  compared  with  earthworms  (cf .  p.  42).  They 
are  also  of  much  general  interest  because  some  are  what  are 
known  as  ' '  disease-germs. ' '  Most  bacteria,  however,  are  not 
jparasitie^  hvX  saprophytic^  i.e.,  live  upon  dead  organic  matters, 
and  therefore  are  not  merely  harmless,  but  positive]y  usef-ul  in 
rendering  back  to  the  inorganic  world  useless  organic  matters. 
Some  species  such  as  the  vinegar  bacteria  are  commercially 
important. 

In  systematic  botany  bacteria  constitute  a  well-defined  group, 
the  Schizomycetes  {fissio7i-fnngi)^  their  near  allies  being  the 
Cyanoj)hyce(B  or  "  blue -green  algae." 

Morphology.  Under  the  microscope  bacteria  appear  as 
minute  rods  {Bacilli)  (Fig.  98),  balls  {Cocci)  (Fig.  100),  or  spirals 
{Spirilla)  (Fig.  104),  sometimes  at  rest,  but  often,  at  least  in 
the  case  of  the  rods  and  spirals,  in  active  motion.      Little  or  no 

192 


SHAPES  OF  BACTERIA. 


193 


structure  can  be  made  out  in  them  by  the  beginner,  to  whom 
they  usually  appear  at  first  sight  like  pale,  translucent  or  watery 
bits  of  protoplasm.  Investigation  has  shown,  however,  that  they 
possess  a  cell-wall  (probably  composed  of  cellulose)  and  a  ikju- 
homogeneous  protoplasm.  Unlike  Protococcus^  but  like  yeast- 
cells,  the  cells  of  bacteria  contain  no  chlorophyll.     Nuclear  mat- 


Fia.  98.  —  Bacillus  Megaterium= 
Rods  (unstained)  in  various 
aggregations  as  commonly  seen 
with  a  high  powor  after  their 
cultivation  in  bouillon  and 
while  rapidly  Rowing  and  mul- 
tiplying by  transverse  divi- 
sion. 


0 


Fig.  9^.  — Bacilli  from. 
Ha y  I  ifimo n  (t/ nsta in- 
ed).  The  filaments  at 
the  left  in  a  condition 
of  active  vegetation. 
The  middle  filament 
forming  spores.  The 
filament  to  the  right 
contains  five  spores 
enclosed  in  otherwise 
empty  cells,  the  walls 
of  which  bulge,  proba- 
bly from  the  absorp- 
tion of  water. 


ter  is  present,  either  scattered  about,  or,  if  the  views  of  Biitschli 
be  accepted,  composing  most  of  the  protoplasmic  body  itself. 
Many  bacteria  bear  appendages  in  the  shape  of  tiagella  or 
cilia;  but  these  can  only  be  demonstrated  in  special  cases,  and 
by  special  methods.  They  are  believed  to  be  locomotor  organs, 
and  in  some  cases  have  been  seen  in  active  motion  (Fig.    103). 


194  UNICELLULAR  PLANTS. 

The  minuteness  of  bacteria  is  extraordinary.  Many  bacilli  are 
not  more  than  .005  mm.  (-g-yVo  inch)  in  length  or  more  than  .001 
mm.  {yt^wq  iiich)  in  breadth.      Some  are  very  much  smaller. 

Most  bacteria  are  at  some  time  free  forms ;  but  like  other 
unicellular  organisms  many  of  them  have  the  power  to  pass 
from  a  free-swimming  {swariniiig)  into  a  quiescent  {resting) 
condition.  In  the  latter  some  undergo  a  peculiar  change,  in 
which  the  cell -wall  becomes  mucilaginous,  and  by  the  aggrega- 
tion of  numerous  indiyiduals  or  by  repeated  division  lumjDs  of 
jelly-like  consistency  {zoogloea)  arise.  If  the  jelly  mass  takes 
the  shape  of  a  sheet  or  membranous  skin  (as  haj)23ens  in  the 
mother- of -vinegar),  it  is  sometimes  described  as  Mycoderma 
{fungus-skin)  (Fig.  102). 

Reproduction.  The  bacteria  increase  in  numbers  solely  by 
transverse  division.  Growth  takes  place  and  is  followed  by  trans- 
verse division  of  the  original  cell,  usually  into  halves.  Each  half 
then  likewise  grows  and  divides  in  its  turn.  In  this  way  multi- 
phcation  may  go  on  in  geometrical  progression,  and  with  almost 
incredible  rapidity.  It  has  been  stated  that  such  repeated  divi- 
sions may  follow  only  an  hour  apart,  and  on  this  basis  it  is  easy 
to  compute  the  enormous  numbers  to  which  a  single  cell  may 
give  rise  in  a  single  day. 

If  separation  after  division  is  complete,  strictly  unicellular 
forms  arise.     If  actual  separation  is  postponed,  long  rods,  chains, 

or  plates  (in  the  case  of  cocci) 


,i^"'h. 


'••f  •••"y 


'At."**'* 


o 


may  appear.     Different  names 
are  given  to  the  resulting  forms. 
Streptococcus  is   a  moniliform 
or  necklace-like  arrangement; 
Stajyhylococcus^    single    cocci ; 
,   ^       Dijjloccoccus,    cocci   in   jDairs; 
(^  ^v-^    Lejytothrix^      a     filament      of 
/    ]  ^\" — '    bacilli;    Sarcina,    a    plate    of 
O  (3      <20cci  resembling  a  card  of  bis- 
cuit,   or    two    or    more    cards 

Fig.     100— Micrococci    Fig.    101.— Short 
(unstained)  from  hay       Bacilli         (un-    SUperpOSed  ;   CtC. ,   CtC. 

infusion.  stained)   from         Spores.     Soiiie  bacteria  pro- 

hay  infusion.  ^ 

duce    so-called    sjpores    [endo- 
sjpo^^es)   in   the    following   way :       The   contents   of    the    cell 


^^^»j{. 


.AN.V.- 


'->!»•.•.• 


SPORES  OF  BACTERIA. 


195 


Fig.  102.— The  Mother-of- 
Vinegar.  The  edge  of  a 
film  of  zoogloea  of  mother- 
of-vin?gar  as  it  appears 
under  a  high  power.  The 
bajteria  are  seen  imbedded 
in  the  jelly  which  they 
have  secreted. 


withdraw  from  the  wall  and  condense  into  a  (usually  oval) 
mass  at  one  end  of  the  cell,  leavino^  the  rest  of  it  empty 
It  is  at  this  time  that  the  cell-wall 
is  best  seen.  The  condensed  mass 
now  becomes  dark  and  opaque,  appa- 
rently from  the  deposit  upon  itself  of  a 
greatly  thickened  and  peculiar  wall;  it 
refuses  to  absorb  stains  which  the  origi- 
nal cell  would  have  taken,  and  becomes 
exceedingly  resistant  to  extremes  of 
heat,  cold,  and  dryness  (Fig.  105).  To 
these  spores  the  Germans  give  the 
excellent    term    Dauersporen,    i.e., 

enduri7ig 
spores^ 
often  called 

resting  spores.  When  brought  under 
favorable  conditions,  these  sprout 
and,  the  ordinary  bacterium  cell 
having  been  produced,  growth  and 
fission  proceed  as  before.  Obviously 
these  spores  are  very  different  in 
function  from  those  of  Pteris  (p. 
130),  since  they  are  protective 
merely,  and  not  rej^roductive.  They 
correspond,  doubtless,  to  that  phase 
of  animal  life  which  is  known  as  the 
''encysted"  state.  Another  mode 
of  spore-formation  in  bacteria  is  that 
known  as  the  production  of  arthro- 
spores^  in  which  a  long  slender  cell 
mav  become  constricted  and  detach 
daughter-cells  from  one  or  both  ends. 
This  is  obviously  a  sjiccial  case  of 
Fig.  ms.-Ciiiated  Bacteria.    The  unequal  ccll-division,  but  if  it  exists 

c1?r;"^:o:''fspecfren'p"I  at  all  (which  has  been  a„uhted)  it 
by  s.  c.  Keith,  Jr.  Drawn  by  J.  H.   clearly     approaches     agamogenesis 


Emerton.) 


in  such  forms  as  Pteris. 


Physiology.   Income;,  Metabolism,  and   Outgo.       The    bacteria 


196  VNICELLULAR  PLANTS. 

show  a  surprising  diversity  in  the  precise  conditions  of  their 
nutrition,  and  it  is  tlierefore  difficult  to  make  for  them  a 
satisfactory  general  statement.  As  a  group,  however,  and  dis- 
regarding for  the  moment  certain  important  exceptions,  they  are 
to  be  regarded  as  colorless  plants  living  for  the  most  part  upon 
complex  organic  compounds  from  which  they  derive  their  in- 
come   of   matter  and  energy  and  wliich   they  decompose  into 

simpler  compounds  poorer  in  poten- 
:^:  tial  energy.  In  so  doing  they 
i^l  bring  about  certain  chemical 
:/:  changes  in  the  substances  upon 
>.V:  which  they  act  which  are  of  the 
vy.  highest  theoretical  interest,  and 
■^  sometimes  of  great  practical  im- 
%  portance.  Perhaps  the  most  pecul- 
■i:  iar  feature  of  the  physiology  of 
%  bacteria  is  the  fact  that  while  tliey 
;i:'        are  themselves  individuallv  invisi- 

•  ..■  «/ 

^;v;:^:•.:.^>:•.V■:^^•:••v.•:vv/-•;:•::v;::^■^^        ble,  they  collectively  produce  very 
Firiw!-spirnium'nnduia^  couspicuous  and  important  changes 

bacteria  deeply   stained.      Drawn    yh    their   environment.       For  CXaui- 
from  the  first  photographic  repre-  .  i        ,       •  . 

sentation   of  bacteria  ever  pub-  ple,    Vinegar     bacteria    act    upon 
lished,  viz.,  that  of  Robert  Koch,  alcohol  (in   cidcr,   etc.)  and   by  a 

in  Cohn's  Beitmye,  1876.)  \        ...  i        i 

process  oi  oxidation  slowly  convert 
it  into  acetic  acid  and  water,  thus : — 

C,H.O  +  O,  =  C,H.O,  +  H,0. 

Here  it  is  not  the  bacteria  that  are  most  conspicuous,  but  the 
effect  which  they  produce.  It  is  clear  that  the  alcohol  can  be 
only  one  factor  in  the  nutriment  of  the  organism,  because  it 
contains  no  nitrogen,  and  the  above  reaction  cannot  represent 
more  than  a  phase  in  the  nutrition  of  the  bacterium.  That  this 
is  indeed  the  case  is  proved  by  the  fact  that  if  the  conditions  be 
somewhat  changed  the  same  bacteria  may  go  further  and  convert 
the  acetic  acid  itself  into  carbonic  acid  and  water : — 

C,H,0,  +  O,  =  2C0,  +  2H,0. 

Chemical  changes  of  this  kind  in  which  the  effect  upon  the  en- 


FERMENTS  AND  FERMENTATION. 


197 


vironment  is  more  conspicuous  than,  and  out  of  all  proportion  to, 

the  change  in  the  agent  are  in  some  cases  known  as  ferinen- 

tations^  and  the  agent  effecting  the  change  is  described  as   a 

ferment.     Some  ferments  are  organized  or  living^  and  some  are 


0 


Fig.  105.— Bacillus  megaterium  (x  600).  Spore  formation  and  germination.  .4, 
a  pair  of  rods  forming  spores,  about  3  o'clock  p.m.  B,  the  same  about  an  hour 
later.  C,  one  hour  later  still.  The  spores  in  C  were  mature  by  evening  ;  the  one 
apparently  begun  in  the  third  upper  cell  of  A  and  B  disappeared  ;  the  cells  in  0 
which  did  not  contain  spores  were  dead  by  9  p.m.  D,  a  five-celled  rod  with  three 
ripe  spores,  placed  in  a  nutrient  solution,  after  drying  for  several  days,  at  12.30, 
P.M.  E,  the  same  specimen  about  1.30  p.m.  F,  the  same  about  4  p.m.  G,  a  pair  of 
ordinary  rods  in  active  vegetation  and  motion.    (After  De  Bary.) 

unorganized  or  lifeless.  Of  the  former  the  vinegar  bacterium 
and  yeast  are  good  examples.  Of  the  latter  the  digestive  fer- 
ments, like  pepsin^  jptyalin.,  and  trypsin^  and  certain  vegetal 
ferments,  like  diastase  of  malt  are  familiar  instances. 

As  a  rule  the  bacteria  seem  to  prefer  neutral  or  slightly 
alkaline  nitrogenous  foods.  They  therefore  decompose  more 
readily  meats,  milk,  and  substances  (such  as  beef-tea)  made  of 
animal  matters;  less  readily  acid  fruits,  timber,  etc.  If  in  the 
course  of  their  activity  they  decompose  meats,  or  fish,  eggs,  etc., 
with  the  production  of  evil-smelling  gases  or  putrid  odors,  the 
process  is  known  2i^  piotref action.  Rarely,  bacteria  invade  the 
animal  (or  plant)  body  and  act  upon  the  organic  matters  which 
they  find  there.  In  such  cases  disease  may  result,  and  the 
bacteria  concerned  are  then  known  as  disease  germs. 

But  while  bacteria  appear  to  prefer  highly  organized  nitrog- 
enous (proteid)  food,  they  are  by  no  means  dependent  upon  it. 
Experiments  have  shown  that  many  species  can  thrive  upon 
Pasteur's  fluid,  a  liquid  containing  only  ammonium  tartrate  and 
certain  purely  inorganic  substances;  and  one  bacterium,  at  least 
(the  "nitrous"),  according  to  Winogradsky,  can  thrive  upon 
ammonium  carbonate.  If  this  proves  to  be  true  for  other  spe- 
cies, it  will  show  that  bacteria  can  not  only  obtain  their  nitrogen 
from  the  inorganic  world,  but  their  carbon  also.      Enough  has 


198  UNICELLULAR  PLANTS. 

been  said  already  to  prove  tliat  the  bacteria  are  plants,  for  only 
plants  can  live  upon  inorganic  food.  But  if  the  ex23eriments 
just  referred  to  are  correct,  bacteria  are  not  only  plants,  but,  in 
spite  of  their  lack  of  chlorophyll,  some  at  least  appear  to  be 
able,  like  green  plants  to  manvfacture  their  own  food  out  of 
the  raw  materials  of  the  morganic  world.  The  importance  of 
this  fact  in  studies  of  the  genealogy  of  organisms  is  very  great, 
for  we  are  no  longer  obliged  to  suppose  all  chloroj^hylless  plants 
to  be  degenerate  forms.  They  may  have  been  the  primitive 
forms  of  life. 

As  was  the  case  with  yeast  and  Protococcus^  it  is  extremely 
difficult  to  make  any  precise  statement  concerning  the  income  or 
outgo  of  bacteria.  It  is  believed,  however,  that  the  income 
always  includes  salts  and  water,  and  the  outgo  C02,H20  and 
some  nitrogenous  compound  or,  possibly,  free  (dissolved)  nitro- 
gen. In  more  favorable  cases  the  income  appears  to  include 
proteids,  fats,  and  carbohydrates  or  their  equivalents.  Sugar  is 
freely  used  under  some  circumstances ;  and  fats  (when  saj^onified) 
and  proteids  peptonized,  or  otherwise  altered,  might  readily  be 
absorbed.  It  is  probable  that  soluble  ferments  are  excreted  by 
the  bacteria,  which  dissolve,  and  make  absorbable,  solid  matters, 
such  as  meat  or  white  of  ^^^ ;  and  if  this  is  true,  bacteria  exhibit 
a  kind  of  external  digestion.  However  this  may  be,  it  is  certain 
that  bacteria  can  live  and  multiply  upon  an  amount  of  food  ma- 
terials so  small  as  almost  or  quite  to  elude  chemical  analysis ;  and 
it  is  fair  to  say  that  they  are  among  the  most  delicate  of  all 
reagents. 

It  must  not  be  inferred  from  what  has  been  said  above  that  bacteria  are 
always  oxidizing  agents.  Broadly  speaking  and  in  the  long  run  they  are 
such,  and  in  this  respect  they  resemble  animals.  Like  the  latter  they  are 
unable  (because  of  want  of  chlorophyll)  to  utilize  solar  energy,  and  there- 
fore must  obtain  their  energy  by  oxidizing  their  food.  Yet  under  certain 
circumstances  bacteria  act  as  reducing  agents,  as,  for  example,  when  they 
reduce  nitrates  to  ammonia.  This  action  only  takes  place,  however,  in 
the  presence  of  organic  matter,  and  appears  to  be  merely  an  incidental 
effect,  the  oxygen  of  the  nitrate  being  needed  for  the  oxidation  of  carbon. 
What  at  first  sight  appears  to  be  an  exception,  therefore,  proves  in  the  end 
to  be  a  part  of  a  general  law  that  bacteria,  like  animals,  are  oxidizing 
agents,  are  dependent  for  their  energy  upon  the  potential  energy  of  their 
foods,  and  are  unable  to  utilize  solar  energy  (p.  104). 


METHODS  OF  STERILIZING.  199 

It  has  recently  been  shown  that  many  bacteria  under  circumstances 
otherwise  favorable  are  killed  by  exposure  to  sunlight. 

Related  Forms.  According  to  our  present  ideas  of  classification  the 
bacteria  form  a  somewhat  isolated  group,  their  nearest  relatives  being  the 
slitne-moulds  {Myxomycetes)  and  especially  the  Mijxobacteria  of  Thaxter,  on 
the  one  hand,  and  the  CyanopJiycece  the  "blue-green"  or  "fission"  algae 
on  the  other.     Neither  of  these,  however,  need  be  considered  here. 

Why  Bacteria  are  Considered  to  be  Plants.  The  bacteria  were 
formerlj  regarded  as  infusorial  aninialcules  (because  they  abound 
in  infusions,  and  many  have  the  power  of  active  m(jveiiient). 
They  are  still  regarded  by  some  as  animals.  Most  ])iologists, 
however,  regard  them  as  plants,  because  they  can  live  without 
proteid  food  (which  no  animal,  so  far  as  known,  can  do),  and 
because  in  their  method  of  reproduction  and  in  their  growtli- 
forrns  they  more  nearly  resemble  the  Cydnojjhycece  than  they  do 
any  animal.  There  is  also  reason  to  think  that  their  cell- wall  is 
composed  of  cellulose. 

Bacteria  and  their  Environment.  The  relations  of  organisms  to  tem- 
perature and  moisture  have  been  more  thoroughly  studied  for  the  bacteria 
than  for  any  other  unicellular  organisms  on  account  of  tlieir  bearing  upon 
modern  theories  of  infectious  disease.  In  general,  temperatures  above 
70°  C.  are  fatal  to  ordinary  bacteria.  In  general,  as  is  shown  by  common 
experience  with  the  "keeping"  of  foods  in  cold  storage,  bacteria  are  be- 
numbed but  not  killed  by  moderate  cold.  But  in  special  cases,  particu- 
larly when  they  are  dried  slowly,  bacteria  may  withstand  even  prolonged 
boiling  or  freezing  or  the  action  of  poisons,  so  that  the  removal  or  destruc- 
tion of  the  last  traces  of  bacterial  life  is  often  very  difficult. 

Sterilization  and  Pasteurizing.  The  removal  of  all  traces  of  living 
matter  from  any  substance,  and  in  particular  the  destruction  of  all  bac- 
terial life,  is  known  as  sterilization.  To  free  organic  substances  from  the 
larger  forms  of  life  is  a  comparatively  easy  matter;  but  bacteria  are  so 
minute  and  so  ubiquitous  that  scarcely  anything  is  normally  free  from 
them,  and  they  are  so  hardy  that  it  is  exceedingly  difficult  to  destroy  them 
without  at  the  same  time  destroying  the  substances  which  it  is  desired  to 
sterilize.  They  are  not  normally  present  in  the  living  tissues  of  plants  or 
animals  which  are  sealed  against  their  entrance  by  skins  or  epitiielia  ;  but 
after  these  are  broken  or  cut  open  (as  in  wounds)  bacteria  speedily  invade 
the  tissues.  Ordinary  earth,  as  has  been  said  above,  teems  with  bacteria, 
which  are  easily  dried  and  disseminated  in  dust  driven  by  the  wind.  What- 
ever is  in  contact,  therefore,  with  the  air  or  exposed  to  dust  or  dirt  is  never 
free  from  bacteria,  and  meat  or  milk  which  in  the  living  animal  are  nor- 
mally sterile,  if  exposed  to  the  air  soon  become  contaminated  with  bacteria. 
Sterilization  (such  as  is  required  to  preserve  canned  goods,  for  example) 


200  UNICELLULAR  PLANTS. 

may  be  effected  by  heat  and  continued,  after  cooling,  by  exclusion  of 
germ -laden  air.  Disinfection,  which  is  the  destruction  of  bacterial  life  by 
powerful  poisons,  is  another  form  of  sterilization.  Still  another  is  filtra- 
tion through  media  impervious  to  germs,  such  as  occurs  in  the  well- 
known  clay,  or  porcelain,  water-filters.  In  the  last  case  the  pores  of  the 
filter  are  large  enough  to  allow  the  water  very  slowly  to  pass,  but  too  small 
for  the  bacteria. 

In  some  cases,  especially  those  in  which  disease-producing  (j9(7^^05re???!c) 
germs  may  be  present  and  yet  it  is  impossible  to  use  poisons  and  undesira- 
ble to  use  a  high  temperature.  Pasteurization  is  resorted  to.  This  con- 
sists in  heating  to  a  temperature  (usually  75^  C.)  high  enough  to  destroy 
the  particular  pathogenic  germs  supposed  to  be  present,  but  not  high 
enough  to  alter  the  digestibility  or  other  valuable  properties  of  the  liquid 

in  quostion. 

For  the  medical,  economic,  and  sanitary  aspects  of  problems  relating 
to  the  Imcteria,  reference  must  be  had  to  the  numerous  treatises  upon 
Bacteriology,  perhaps  the  youngest,  and  certainly  one  of  the  most  impor- 
tant, of  the  biological  sciences. 


J      i 


CHAPTEE  XYII. 

A  HAY  INFUSION. 

If  a  wisp  of  liaj  is  put  into  a  beaker  of  water  and  the  mix- 
ture allowed  to  stand  in  a  warm  place  there  is  soon  formed  what 
is  known  as  a  hay  infusion.  Microscopical  examination  of  a 
drop  of  the  liquid  at  the  end  of  the  first  hour  or  two  reveals 
little  or  nothing,  and  if  the  beaker  be  held  up  to  the  liglit  the 
liquid  appears  clear  and  bright.  But  after  some  hours  a  marked 
change  is  found  to  have  taken  place.  The  liquid,  originally 
clear,  has  become  cloudj,  and  a  drop  of  it  examined  microscop- 
ically wall  be  found  to  be  swarming  with  bacteria.  A  day  or 
two  later,  the  cloudiness  meanwhile  increasing,  the  microscope 
generally  reveals  not  only  swarms  of  bacteria,  but  also  numerous 
infusoria.  At  the  same  time  the  color  of  the  liquid  has  deep- 
ened, it  begins  to  appear  turbid,  a  scum  forms  on  the  surface, 
and  the  odor  of  hay,  which  was  present  at  the  outset,  is  replacec* 
by  the  less  agreeable  odors  of  putrefaction.  The  simple  ex- 
periment of  bringing  together  hay  and  water  has,  in  fact,  set  in 
motion  a  complicated  series  of  physical,  chemical,  and  biological 
phenomena. 

The  Composition  of  a  Hay  Infusion.  A  hay  infusion  consist? 
of  two  principal  constituents,  hay  and  water.  But  neither  i)\ 
these  is  chemically  pure.  Hay  is  only  dried  grass  which  for 
weeks,  and  even  months,  was  exposed  in  the  field  to  wind  and 
dust.  Covered  with  the  latter — often  the  pulverized  mud  of 
roads  and  roadside  pools — hay  is  richly  laden  with  dried  bacteria 
and  other  micro-organisms;  while  water,  such  as  is  ordinai-ily 
drawn  from  a  tap,  frequently  contains  not  only  an  abundance  of 
free  oxygen  and  various  salts  in  solution,  but  also  numerous  bac- 
teria, infusoria,  algae,  diatoms,  and  other  micro-orgamsnis  iu 
suspension.  In  the  making  of  a  hay-infusion,  therefore,  numer- 
ous factors  co-operate,   and   a   series  of   complicated    reactions 

follow  one  another  in  rapid  succession.       At    the   start    both 

201 


202  A  HAT  INFUSION. 

liaj  and  water  are  in  i  state  of  comparative  rest  or  equilib- 
rium, but  upon  bringing  them  together  action  and  reaction 
begin.  First,  the  dust  on  the  hay  is  wetted  and  soaked, 
and  any  micro-organisms  in  it  or  adliering  to  the  hay  are  set  free, 
and  float  in  the  water ;  next,  the  water  finds  its  way  into  the 
stems  and  leaves  of  the  hay,  causing  them  to  swell  and  resume 
their  orio;inal  form.  At  the  same  time  various  soluble  constitu- 
ents  of  the  daad  grass,  such  as  salts,  sugars,  and  some  nitrog- 
enous substances,  diffuse  outward  into  the  water,  while  from 
such  cells  as  have  been  crushed  or  broken  open  during  drying 
or  handling,  solid  proteid  or  starchy  substances  may  pass  out  and 
mingle  with  the  water.  These  simple  physical  reactions  obvi- 
ously involve  a  disturbance  of  the  chemical  equilibrium  of  the 
water.  Originally  able  to  support  only  a  limited  amount  of  life 
(such  as  exists  in  drinking-waters),  it  is  now  a  soil  enriched 
by  what  it  has  gained  from  the  hay.  The  bacteria,  extremely 
sensitive  to  variations  in  their  environment,  and  especially  to 
their  food-supply,  immediately  proceed  to  multiply  enormously, 
so  that  a  biological  reaction  follows  closely  on  the  heels  of  the 
chemical  change.  But  as  a  result  of  their  metabolic  activity  the 
bacteria  set  up  extensive  chemical  changes,  which  in  their  turn 
involve  physical  disturbances.  For  example,  the  dissolved  oxy- 
gen with  which  the  liquid  was  saturated  soon  disappears,  so  that 
more  oxygen  must,  therefore,  diffuse  into  the  liquid  from  the 
atmosphere.  Carbonic  acid  is  generated  in  excess,  and  some 
may  pass  outwards  to  the  air.  Also,  as  a  result  of  the  vital 
activity  of  the  micro-organisms  the  temperature  of  the  infusion 
may  rise  a  fraction  of  a  degree  above  that  of  the  surround- 
ing atmosphere. 

We  are  concerned,  however,  chiefly  with  the  biological 
results.  In  consequence  of  the  exhaustion  of  the  oxygen  supply 
in  the  lower  parts  of  the  liquid,  many  of  the  bacteria  which 
require  abundant  oxygen  for  their  growth  {aerobes)  find 
their  way  to  the  surface,  where  some  pass  into  a  kind  of 
resting  stage  {zoogloea)  and  form  a  scum  or  skin  {mycoclerm)  on 
the  surface  of  the  liquid.  Others,  for  which  free  oxygen  is  not 
necessary  or  to  which  it  is  even  prejudicial  {anaerobes)^  live  and 
thrive  in  the  deeper  parts  of  the  beaker.     But,  meantime,  an- 


PHYSIOLOGICAL    CYCLE  IN  THE  INFUSION.  203 

other  plienoinenon  lias  occnrred.  The  infusijria,  originallv  few 
in  number,  iiiiding  the  conditions  favorable,  liave  niuhi[)liLMl 
enormously,  and  after  a  day  or  two  may  be  seen  dartini^  in  jind 
out  among  the  bacteria,  especially  near  the  surface,  and  feeding 
upon  them.  Among  the  infusoria,  liowever,  are  some  wliich 
feed  upon  their  fellows,  so  that  we  soon  have  the  herbivorous 
infusoria  pursued  by  carnivorous  forms,  the  whole  scene  illus- 
trating in  one  Held  of  the  microscope  that  struggle  for  existence 
which  is  one  of  tlie  fundamental  facts  of  biology. 

Obviously,  this  chain  of  life  is  no  stronger  than  its  weakest 
part.  The  hay  is  the  source  of  the  food-su])ply  for  all  these 
forms,  and  this  supply  must  eventually  become  exhausted. 
"When  this  happens,  the  bacteria  cease  to  multiply,  the  herbivo- 
rous infusoria  which  depend  upon  them  perish  or  pass  into  a  rest- 
ing stage,  the  carnivorous  infusoria  likewise  starve,  and  all  the 
biological  phenomena  must  either  come  to  an  end  or  change 
their  character. 

Up  to  this  point  the  action  is  purely  destructive.  But  sooner 
or  later  microscopic  green  plants  may  appear  on  the  scene, — 
Protococcits^  it  may  be,  or  its  allies, — and  a  constructive  action 
begin,  the  waste  products  of  the  animals  and  of  the  bacteria  be- 
ing rebuilt  by  the  green  plants  into  complex  organic  matter.  1  )y 
this  time,  also,  the  dissolved  organic  matter  will  have  been 
largely  extracted  from  the  liquid,  the  l)acteria  for  the  most 
part  devoured  by  the  infusoria,  and  the  latter  may  more  or  less 
completely  have  given  way  to  larger  forms — to  rhizopods,  roti- 
fers, small  worms,  and  the  like.  The  putrefying  infusion  has 
run  its  course,  and  the  ordinary  balance  of  nature  has  been 
restored. 

Thenceforw^ard  an  approximate  equilibrium  is  maintained. 
The  green  plants  build  complex  organic  matter  and  store  uj) 
the  energy  of  light.  The  animals  feed  upon  the  j)lants,  or 
upon  one  another,  break  down  the  complex  matter,  and  dissi- 
pate energy.  The  ever-present  bacteria  break  down  all  the 
refuse,  extract  soluble  organic  matter  from  the  water,  decom- 
pose the  dead  bodies  of  the  animals  or  plants,  and  in  the  end, 
it  may  be,  themselves  fall  victims  to  devouring  infusoria.  The 
physiological  cycle  is  complete. 


204  A  HAT  INFUSION. 

A  liaj  infusion  tlius  affords  in  miniature  a  picture  of  the  liv- 
ing world.  The  green  plants  are  constructive,  and  in  the  sun- 
light build  up  matters  rich  in  potential  energy.  These  as  foods 
support  colorless  plants  (such  as  bacteria)  or  animals.  On  these, 
again,  herbivorous  and  carnivorous  animals  feed ;  and  so,  in  the 
world  at  large,  as  in  the  hay  infusion,  omnivorous  as  well  as 
carnivorous  animals,  in  the  long  run,  feed  upon  herbivorous 
animals,  and  the  latter  upon  plants — either  colorless  or  green — 
which  thus  stand  as  the  bulwark  between  animals  and  starvation. 


APPENDIX. 


SUGGESTIONS    FOE    LABOEATORY   STUDIES    AND 

DEMOXSTEATIONS. 

The  "Laboratory  Directions  in  General  Biology,"  published 
and  copyrighted  by  Prof.  E.  A.  Andrews  of  Jolms  Hopkins 
University,  will  be  found  extremely  useful  and  ijractlcal.  Also 
the  following  :  Huxley  and  Martin's  "Practical  Biology  "  (Howes 
and  Scott),  and  the  accompanying  ' '  Atlas  of  Biology,  • '  by  Howes ; 
Marshall  and  Hurst's  "Practical  Zoolofirv,"  Colton's  "Practical 
Zoology,"  Bumpus's  "Invertebrate  Zoology,"  Dodge's  "Ele- 
mentarv  Practical  Bioloo-v,"  Brooks's  "Handbook  of  Inverte- 
brate  Zoology."  According  to  our  experience,  the  periods  for 
the  course  should  be  so  arranged  as  to  afford  laboratory  work 
and  recitations  or  quizzes  in  about  the  proportions  of  three  to 
two  (for  examj)le,  three  periods  of  laboratory  work  and  demon- 
stration to  two  of  quiz),  for  a  half-year. 

Chapter  I.     (Introductory.) 

It  is  convenient  to  give  at  the  outset  one  or  more  practical 

lessons  on  the  microscope,  affording  the  student  an  opportunity  to 

learn  its  different  parts,  use  its  adjustments,  test  the  magnifying 

power  of  the  various  combinations,  etc.      A  good  object  for  a 

first  examination  is  a  human  hair,  which  serves  as  a  convenient 

standard  of  size  for  comparison  with  other  things.      Other  good 

objects  are  starches,  the  scales  from  a  buttertly's  wing  (sketch 

under  different  powers),  a  drop  of  milk  or  blood,  and  ]i(nv(h^rod 

carmine  or  gamboge  rubbed  up  in  water  (to  show  the  Brownian 

movement).      The  student  should    compare   the  same  object  as 

seen  under  the  simple  and  the  compound  microscope  (to  show 

205 


206  APPENDIX. 

reversal  of  the  image  in  the  latter),  and  should  during  the  course 
learn  the  nse  of  the  camera  kicida  (Abbe's  camera,  of  Zeiss,  tlie 
best).  The  stage-micrometer  may  also  be  examined  at  this  time 
or  later,  and  the  student  taught  to  prepare  a  scale  (see  Andrews) 
by  drawing  the  lines,  with  camera,  on  a  card  under  different 
p>owers  (A  +  2,  D  +  2,  D  +  4,  of  Zeiss),  and  labelhng  each 
with  the  names  of  lenses  and  actual  size  of  the  spaces,  as  stated 
on  the  micrometer. 

Pencil- drawing  should  begin  as  soon  as  the  first  sj^ecimen  is 
in  focus,  and  sketches  should  be  made,  from  the  very  first  exercise 
onward,  of  everything  really  studied.  It  is  absolutely  indis- 
pensable to  heej)  a  laboratory  note-hooh^  which  ought  at  any  time 
to  give  tangible  evidence  that  the  laboratory  study  is  bearing 
fruit ;  and  in  the  very  first  laboratory  exercise  a  beginning  should 
be  made  in  this  direction. 

The  preliminary  microscopy  of  one  or  two  laboratory  peri- 
ods, corresponding  to  the  time  spent  in  conferences  U2)on  the  first 
chapter  of  the  text-book,  leads  naturally  up  to  the  easy  micro- 
scopical studies  required  in  connection  with  the  second  chapter. 

ChAPTEK    II.        (STilUCTURE    OF    LiVING    ORGANISMS.) 

The  laboratory  work  may  be  made  very  brief  and  simple, 
and  the  facts  shown  largely  by  illustration.  Tlie  principal 
organs  of  a  j)lant  and  of  a  five  or  dissected  animal  may  be  shown 
and  some  of  the  more  ob^dous  tissues  pointed  out.  A  frog  under 
a  bell-glass,  and  a  flowering  plant  (geranium)  in  blossom,  j)laced 
side  by  side  on  the  demonstration-table  will  serve  to  suggest 
materials  for  the  lists  of  organs  and  the  comparisons  called  for. 

The  skin  of  a  Calla  leaf  is  easily  stripjDed  off  and  demon- 
strated to  the  naked  eye  as  one  form  of  tissue.  It  may  then  be 
cut  up  and  distributed  for  microscopic  study  and  for  j^roof  that 
it  is  composed  of  cells.  (During  this  process  air  is  apt  to  replace 
water  lost  by  evaporation,  and  must  be  displaced  by  alcohol, 
which  in  turn  must  be  removed  by  w^ater.) 

For  a  first  microscopical  examination  of  tissue  there  is  no 
better  object  than  the  leaf  of  a  moss  (a  species  having  thin  broad 
leaves  should  be  chosen)  or  a  fern  prothalHum.  Otlier  good 
objects  are  thm  sections  of  a  potato-tuber  from  just  helow  the 


LABORATORY  STUDIES  AND  DEMONSTRATIONS.        207 

surface  (stained  witli  dilute  iodine  to  sliow  nnolei  and  ^itarcli- 
grains),  and  frou-'s  or  newt's  l>lood,  mixed  with  nuniial  salt  scJu- 
tion,  and  examined  either  fresh  or  slightly  stained  with  dilute 
iodine. 

Thin  sections  of  pith  (elder,  etc.),  from  which  the  air  lias 
been  displaced  by  alcohol,  give  good  pictures  of  tissue  c<>jn]>used 
of  empty  cells.  Fresh  or  alcoholic  nniscle  from  the  frog's  leg, 
gently  teased  out,  shows  muscular  tissue  to  be  composed  of  elon- 
gated cells  (iihres).  Finally,  the  student  may  prove  that  he 
Inmself  is  composed  of  cells  by  gently  scraping  the  inside  of  his 
lip  or  cheek  with  a  scalpel,  mounting  the  scrapings  on  a  slide, 
and  after  adding  a  drop  of  Delalield's  h^ematoxylin,  covering, 
and  examining  in  the  usual  way. 

To  show  the  lifeless  matter  in  living  tissue  it  suffices  to  ex- 
amine frog's  blood  or  human  blood;  sections  of  potatoes,  es- 
pecially if  lightly  stained  wdth  iodine ;  sections  of  geranium  stems. 
{Pelargonium)^  which  usually  show  crystals  in  some  of  the  more 
peripheral  cells ;  cartilage,  stained  with  iodine,  in  which  the  life- 
less matrix  remains  uncolored ;  or  prepared  sections  of  bone,  in 
which  the  spaces  once  filled  by  the  living  cells  are  now  black  and 
opaque,  being  filled  with  dust  in  the  grinding,  or  with  air. 

Chapter  III.      (Protoplasm  and  the  Cell.) 

Naked-eye  Examination  of  Protoplasm.  A  drop  of  ]u'oto- 
plasm  is  readily  obtained  from  one  of  the  long  (internodal)  cells 
of  Nitella^  after  removing  the  superfluous  water  and  snipping  off 
one  end  of  the  cell  with  scissors.  The  cell  collapses  and  the 
drop  forms  at  the  lower  (cut)  end.  It  may  be  transferred  to  a 
(dry)  slide  and  tested  for  its  viscidity  by  touching  it  with  a 
needle.  Microscopically  it  is  instructive  chiefly  by  its  lack  of 
marked  structure. 

The  Parts  of  the  Cell.  The  structure  of  the  cell  is  beauti- 
fully shown  in  properly  stained  and  mounted  preparations  of  un- 
fertilized star-fish  or  sea-urchin  eggs,  or  of  apical  buds  of  j^iUUa. 
If  these  are  not  availal)le  potato-cells  or  cartilage  cells  do  very 
well;   or  sections  of  epithelium,  glands,  etc.,  may  be  shown. 

The  class  may  also  mount  and  draw  frog's  or  newt's  blood- 
cells,  prepared  and  double-stained  as  follows.    The  blood  is  spread 


208  APPENDIX. 

out  evenly  on  a  slide  and  dried  cautiously  over  a  flame.  Stain 
witli  hsematoxylin  for  three  minutes ;  wash  thoroughly  with  water, 
add  strong  aqueous  solution  of  eosin,  allow  to  stand  one  minute ; 
wash  this  time  very  rapidly,  remove  the  excess  of  water  quicMy 
with  filter-paper  pressed  down  over  the  whole  slide ;  dry  rapidly, 
and  examine  with  low  power.  If  successful  mount  in  balsam ;  if 
the  specimen  is  not  pink  enough  add  more  eosin  and  wash  still 
more  rapidly  than  before.  In  good  specimens  the  cells  keep 
tlieir  form  perfectly,  the  cytoplasm  is  briglit  pink,  and  the  nucleo- 
plasm is  light  purple. 

Epidermis  from  young  leaves  of  hot-house  lilies  (''  African  " 
lily,  ' '  Chinese  ' '  lily,  and  especially  lily-of -the-valley)  yields 
cells  showing  finely  the  cell-wall,  nucleus,  and  (in  favorable 
cases)  cytoplasm.  If  stained  with  acetic  acid  and  methyl-green 
the  nuclei  are  highly  colored;  with  Delafield's  hsematoxylin  the 
cytoplasm  is  more  easily  seen. 

Cell-divisions  or  Cleavage  are  easily  observed  in  segmenting 
ova  or  in  fresh  specimens  of  Protococcus  (Pleurococcus)  de- 
taclied  from  moistened  pieces  of  bark  which  bear  these  algge. 
(See  p.  178). 

Stages  in  the  cleavage  of  the  ovum  may  be  seen  in  the  seg- 
menting eggs  of  fresh-water  snails  {Pliysa^  Planorhis)  which 
are  easily  procured  at  almost  any  tune  by  keeping  the  animals  in 
aquaria.  The  old  egg-masses  should  be  removed  so  as  to  ensure 
the  eggs  being  fresh.  Or  a  supply  of  preserved  segmenting  eggs 
(star-fish,  sea-urchin)  may  be  kept  for  demonstrating  the  early 
stages. 

Protoplasm  in  Motion.  The  best  introduction  to  j)rotoplasm 
in  motion  is  afforded  by  a  su23erficial  examination  of  Amoeba 
(for  procuring  A?nceha  see  above.  Chapter  XII).  If  A^inoeba  is 
not  available  young  living  tips  of  Nitella  or  Chara  may  be  used. 
Anacharis  and  Tradescantia  are  useful,  and  often  very  beautiful, 
l)ut  less  easy  to  manage,  as  a  rule.  In  mounting  Nitella  or 
Chara  care  must  be  taken  not  to  crush  the  cells,  and  as  far  as 
possible  pale  fresh  specimens  rather  than  darker  and  older  ones 
should  be  chosen.  If  Anacharis  is  to  be  studied  the  youngest 
leaves  should  be  selected  from  the  budding  ends,  and  not,  as  is 
sometimes  recommended,  leaves  which  are  becoming  yellow. 
The  movement  in  the  cells  of  Anacharis  leaves  often  besrins 


LABORATORY  STUDIES  AND  DEMONSTRATIONS.        209 

only  after  tlie  leaf  lias  been  mounted  for  a  lialf-liour  or  more ; 
but  when  once  established  affords  one  of  the  most  beautiful  and 
striking  examples  of  protoplasmic  motion.  If  Tra(.U  Heard  at  is  to 
be  used,  care  must  be  taken  to  have,  if  possi])le,  tiowers  just  open 
or  opening.  The  morning  is  therefore  preferable  for  work  uu 
this  plant.     High  powers  are  necessary. 

In  all  these  forms  the  movements  may  often  be  stinnilated  l)y 
placing  a  lamp  near  the  microscope  or  l)y  cautiously  warming 
the  slide  over  the  lamp-chhnney.  Ciliary  action  is  easily  shown 
in  bits  of  the  gills  taken  from  fresh  clams,  mussels,  or  oysters,  or 
in  cells  scraped  from  the  inside  of  the  frog^s  oesophagus.  A 
striking  demonstration  is  easily  given  by  slitting  open  a  frog's 
(or  turtle's)  oesophagus  lengthwise,  pinning  out  flat,  moistening 
with  normal  salt  solution,  and  placing  tiny  bits  of  moistened  cork 
on  the  surface.  The  progressive  movement  of  the  cork-bits  is 
then  very  obvious.  Muscular  contractility  is  easily  shown  l)y 
removing  the  skin  from  a  frog's  leg,  dissecting  out  the  sciatic 
nerve,  cutting  its  upper  end,  and  then  stinmlating  the  lower  end, 
if  possible,  by  contact  with  a  pair  of  electrodes,  otherwise  by 
pinching  it  with  forceps.  If  the  necessary  apparatus  is  available 
the  regular  muscle-nerve  preparation  may  be  shown  (see  Foster 
and  Langley's  "Practical  Physiology''). 

Food-stuffs  Contain  Energy.  This  may  be  shown  (in  dem- 
onstrations) by  sprinking  finely  ])owdered  and  ihorou^jhl y 
dried  starch,  sugar,  or  flour  upon  a  fire,  or  upon  a  platinum  dish 
or  piece  of  foil  heated  to  redness  over  a  small  flame.  Oils  and 
dried  and  powdered  albumen  (proteid)  may  be  similarly  made  to 
burn  with  almost  explosive  violence  if  apj^lied  in  a  state  of  fine 
division  in  presence  of  air. 

The  Chemical  Basis,  {ci)  Proteids\  Coagulation'^  Blijor  Jfor- 
tis\  Rigor  Caloris.  White-of-egg  may  be  shown  (in  demonstra- 
tion) and  made  to  coagulate  in  a  test-tube  hung  down  into  a 
beaker  of  water  under  which  is  put  a  flame.  A  therm(Mnetcr  in 
the  test-tube  may  be  read  off  from  time  to  time  as  the  ex])eri- 
ment  advances,  until  finally  coagulation  begins,  when  the  temper- 
ature is  noted.  The  death-stiffening  [rigor  rnortin)  comes  on 
very  quickly  in  frogs  killed  with  chloroform.  Ileat-stilfening 
{rigor  caloris)  is  well  shown  by  immersing  one  leg  of  a  deca])i- 
tated  frog  in  a  beaker  of  water  at  40°  C.     The  other  leg  re- 


210  APPENDIX. 

mains  normal  and  affords  a  vahiable  means  of  comparison.  It 
is  not  wortli  wliile  to  make  many  chemical  tests  of  proteids  at 
this  point. 

(b)  Carholiydr cites.  A  useful  demonstration  may  be  made 
of  various  starches,  sugars,  and  glycogen.  The  iodine-test  may 
be  applied  if  desired.  If  time  allows,  the  microscopical  appear- 
ance of  potato-starch,  corn-starch,  Bermuda  arrowroot,  etc., 
may  be  dwelt  upon  in  the  laboratory-work.  Cellulose  is  well 
shown  in  filter-paper  or  absorbent  cotton. 

(g)  Fats.  A  demonstration  of  animal  fats  and  vegetable  oils 
may  be  made  if  time  allows.  They  may  be  examined  microscop- 
ically in  a  drop  of  milk,  in  an  artificial  emulsion  made  by  shak- 
ing u})  sweet  oil  in  dilute  white-of-egg,  or  in  fresh  fatty  tissue 
{from  subcutaneous  tissue  of  mouse,  or  fat-bodies  of  frog).  It  is 
hardly  worth  while  to  examine  these  substances  chemically,  but 
■a  few  simple  tests  may  be  applied  if  desired. 

Dialysis.  A  demonstration  of  dialysis  is  easily  made  by  in- 
verting a  broken  test-tube,  tying  the  membrane  over  the  flaring 
€nd,  filling  the  tube  to  a  marked  point  with  strong  salt  or  glu- 
•cose  solution,  and  immersing  it  in  a  beaker  of  distilled  water. 
After  an  hour  or  so  the  fluid  will  be  found  to  have  risen  in  the 
test-tube  ao^ainst  oTavitv. 

Temperature  and  Protoplasm.  The  profound  influence  of 
temperature  on  protoplasm  is  well  shown  by  the  frog's  heart. 
Decapitate  a  frog  and  destroy  tlie  spinal  cord.  Expose  the 
heart  and  count  tlie  beats  at  the  room  temperature.  Then  pour 
upon  the  heart  iced  normal  salt  solution.  Again  count  the  beats. 
Next  pour  upou  it  normal  salt  solution  heated  to  35°  C.  The 
number  of  beats  will  follow  the  fall  and  rise  of  tempeiTvture. 

Chapters  IY  to  YIII.     (The  Earthworm.) 

Large  earthioorms  must  he  used  or  satisfactory  results  can- 
not be  expected.  Pains  should  therefore  be  taken  to  procure 
the  large  L.  terrestris  {not  the  common  Allolohojpliora  mucosa)^ 
which  is  readily  recognizable  by  the  flattened  posterior  end. 
This  species  is  not  everywhere  common ;  hence  a  supply  should 
be  procured  and  kept  in  a  cool  place  in  barrels  half  full  of  earth, 
on  the  surface  of  which  is  placed  a  quantity  of  moss.      They  will 


LABORATORY  STUDIES  AND  DEMONSTRATIONS.        211 

thus  live  for  inoiitlis.  Z.  Urrestris  mav  be  «jl)t;iinc»l  in  irreat 
numbers  between  April  and  November,  l)y  .searoliiiii:;  for  tlieni 
at  niirht  with  a  lantern  in  localities  where  imnierous  castinors 
show  them  to  abound  (a  rather  heavy  but  rich  s(jil  will  be  found 
most  productive).  They  will  then  be  f(jund  extended  fr(^in  their 
burrows,  lying  on  the  surface  of  the  ground,  and  may  be  seized 
with  the  fingers.  Considerable  dexterity  is  needed,  and  it  is 
necessary  to  tread  very  softly  or  the  worms  take  alarm  and  in- 
stantly withdraw  into  their  burrows. 

For  dissection  fresh  specimens  are  far  preferable  for  most 
purposes,  though  jyroperly  preserved  ones  answer  the  purpose. 
Fresh  specimens  should  be  nearly  killed  by  being  })laced  for  a 
short  time  (about  five  minutes)  in  TO^  alcohol,  and  then  stretched 
out  to  their  utmost  extent  in  50^  alcohol  in  a  dissecting-pan, 
the  two  ends  being  fastened  by  j^ins.  They  should  then  be  at 
once  cut  open  along  the  middle  dorsal  line  with  scissors,  the 
flaps  pinned  out,  and  the  dissection  continued  under  the  50^ 
alcohol.  (They  must  be  comjpletely  covered  with  the  liquid.) 
By  this  method  the  mimitest  details  of  structure  may  l)e  ob- 
served, and  many  of  the  dissections  should  be  done  under  a 
watchmaker's  lens. 

For  preservation  (every  detail  of  which  should  be  attended 
to)  a  number  of  living  worms  are  placed  in  a  broad  vessel  filled 
to  a  depth  of  about  an  inch  with  water.  A  little  alcohol  is  then 
cautiously  dropped  on  the  surface  of  the  water  at  intervals  until 
the  worms  are  stupefied  and  become  perfectly  motionless  and  re- 
laxed (this  may  require  an  hour  or  two).  They  are  then  trans- 
ferred to  a  laro-e  shallow  vessel  containinoj  iust  enouf^h  50^ 
alcohol  to  cover  them,  and  are  carefully  straightened  out  and 
arranged  side  by  side.  After  an  hour  the  weak  alcohol  is  re- 
placed by  stronger  (70^),  which  should  be  changed  once  or  twice 
at  intervals  of  a  few  hours;  they  are  finally  placed  in  9(>^ 
alcohol,  wdiich  should  be  liberally  used.  The  trouble  demanded 
by  this  method  will  be  fully  repaid  by  the  results.  The  W(^run: 
should  be  quite  straight,  fully  extended,  and  plump,  and  they 
may  be  used  either  for  dissection  or  for  microsco2)ic  study. 

For  the  purposes  of  section-cutting  worms  should  be  carefully 
washed  and  placed  in  a  moist  vessel  containing  plenty  of  wet 
filter-paper  torn  into  shreds.      The  worms  will  devour  the  paper, 


212  APPENDIX. 

which  should  be  changed  several  times,  until  the  paper  is  voided 
perfectly  clean.  The  worms  are  then  preserved  in  the  ordinary 
way,  and  when  properly  hardened  are  cut  into  short  pieces, 
stained  with  borax-carmine,  imbedded  in  paraffin,  and  cut  into 
sections  with  the  microtome. 

The  living  worms  should  first  be  observed — their  shaj)e, 
movements,  behavior  to  stimuli,  pulsation  of  the  dorsal  vessel 
(time  the  pulse  and  vary  the  rate  by  temperature  changes). 
"Well-preserved  specimens  should  then  be  carefully  studied  for 
the  external  characters  (draw  through  tlie  fingers  to  feel  the  setse). 
(Sketch.)  Observe  openings.  The  nephridial  openings  cannot  be 
seen,  but  if  preserved  worms  be  soaked  some  hours  in  water  and 
the  cuticle  peeled  off  tliey  may  be  clearly  seen  in  this.  A 
general  dissection  of  a  fresh  specimen  should  now  be  made, 
and  the  positions  of  the  larger  organs  studied.  (Make  partial 
sketch,  to  be  filled  out  afterwards,  as  in  Fig.  24.)  The  alimentary 
canal  and  circulatory  organs  should  now  be  carefully  studied. 
Even  the  smallest  of  the  blood-vessels  may  easily  be  worked  out 
under  the  lens  by  using  fresh  specimens  (killed  in  70^  alcohol 
and  afterwards  dissected  under  water)  and  carefully  turning  aside 
the  alimentary  canal. 

The  alimentary  canal  should  afterwards  be  cut  through  be- 
hind the  gizzard  and  gradually  dissected  away  in  front,  exposing 
the  nerve-cord  and  the  reproductive  organs  (wash  away  dirt  with 
a  pipette).  ISo  great  difficulty  should  be  found  in  making  out  any 
of  the  parts,  excepting  the  testes.  These  are  difficult  to  find  in 
mature  worms,  but  may  be  found  with  ease  in  those  which  have 
no  median  seminal  vesicles  (usually  the  case  with  specimens  hav- 
ing no  clitellum). 

The  contents  of  the  seminal  receptacles  and  vesicles  from  a 
fresh  worm  should  be  examined  with  the  microscope.  Kemove 
an  ovary  (with  forceps  and  small  curved  scissors),  mount  in  water, 
and  study.  (Stained  in  alum-carmine  and  mounted  in  balsam 
the  ovary  is  a  beautiful  object.)  The  student  should  also  re- 
move a  fresh  nephridial  funnel  and  jDart  of  a  nephridium,  and 
study  with  the  microscope.  (This  may  have  to  be  shown  by  the 
demonstrator,  but  should  never  be  omitted,  as  the  ciliary  action 
is  one  of  the  most  striking  things  to  see.)  A  careful  dissection 
of  the  anterior  part  of  the  nervous  system  should  also  be  made. 


LABORATORY  STUDIES  AND  DEMONSTRATIONS.        213 

If  time  presses,  tlie  detailed  study  of  iiiicroscopieal  sections 
may  be  omitted,  but  a  series  of  prepared  sections  shcnild  be  kept 
on  liand  and  a  demonstration  given. 

The  embryological  development  is  too  difficult  to  study,  but 
very  instructive  demonstrations  may  be  given  l>y  those  who  have 
had  some  experience.  In  the  neighborhood  of  Philadelphia  egg- 
capsules  may  be  found  in  great  numbers  in  old  manure-heajis, 
in  May  and  June.  One  end  of  the  capsule  should  be  slicc(|  olf 
with  a  very  sharp  scalpel  and  the  contents  drawn  out,  under 
water,  with  a  large-mouthed  pipette.  The  mass  may  then  be 
mounted  in  water  under  a  supported  cover-glass  and  studied 
with  the  microscope.  The  embryos  may  be  preser\ed  in 
Perenyi's  fluid,  and  either  studied  whole  in  the  preserving  fluid 
or  hardened  in  alcohol  and  cut  into  series  of  sections. 

Chapters  IX  to  XI.      (The  Common  Brake.) 

Except  when  the  ground  is  frozen  Pteris  may  be  dug  up  and 
brouii:ht  into  the  laboratorv  in  a  fresh  state.  Fronds  mav  be 
cut  and  dried  in  midsummer  and  considerably  freshened  (by  a 
moment's  immersion  in  warm  water)  when  needed  to  be  used  (in 
the  oj^ening  exercise)  to  illustrate  the  aerial  portion  of  the  plant. 
Rhizomes  may  be  obtained  at  convenience  and  kept  in  weak 
alcohol  (50^). 

The  Morphology  of  the  Body.  To  illustrate  this,  one  whole 
and  entire  plant  should,  if  possible,  be  at  hand  for  examination. 
The  aerial  and  the  underground  portions  may  then  be  sketched 
in  their  normal  relations.  Branches,  roots,  and  old  leaf-.<talks 
should  be  pointed  out,  identified,  and  sketched. 

The  Anatomy  of  the  Rhizome  should  first  be  made  out  \\\\\\ 
the  naked  eye.  The  lateral  ridges  will  be  detected  by  the  class, 
which  should  be  asked  to  draw  the  cross-section  as  seen  M'ith 
the  naked  eye.  For  this  preliminary  work  each  student  should 
have  a  piece  of  rhizome  two  or  three  inches  in  length.  (Care 
should  afterwards  be  taken  that  the  drawing  has  been  correctly 
placed  dorsoventrally.)  A  rough  dissection  with  jack-knife  or 
large  scalpel  may  next  follow,  with  inferences  as  to  the  characters 
of  the  several  tissues  found  (as  fibrous,  pidpy,  woody,  etc.). 

The  Microscojy'iG  Anatomy  of  the  Bh  home  is  interesting,  and, 


214  APPENDIX. 

for  the  most  part,  easy,  but  demands  mucli  time.  If  time  al- 
lows, cross-sections  of  roots  may  be  made  and  mounted  in  balsam. 
They  are  readily  cut  in  pith.  Sections  of  the  rhizome  may  be 
made  freehand  with  a  razor  or,  better,  with  a  microtome :  but 
the  old  stems  are  exceedingly  hard  and  liable  to  injure  the 
knives. 

The  Frond  or  Leaf  may  be  obtained  in  fruit  in  July  and 
August  and  preserved  in  alcohol.  From  it  sections  of  leaflets 
may  easily  be  got  by  imbedding  in  pith.  Epidermis  is  obtained 
with  some  difticulty  (by  beginners)  after  scraping.  Fresh  fern- 
leaves  from  hothouses  answer  the  purpose  as  well,  are  easier  to 
get,  and  more  attractive.  Really  good  sections  of  fern-leaves  are 
not  easy  for  beginners  to  make.      They  should  be  kept  on  hand. 

Sporangia  may  be  obtained  in  abundance  from  alcoholic 
specimens  of  Pteris^  or  upon  hothouse  ferns,  even  in  midwinter. 
Some  of  the  many  species  of  Pteris  found  in  hot-houses  answer 
every  purpose.  The  thin  edge  of  a  scalpel  slipped  under  tlie.un- 
ri^^e  indusium  removes  the  latter,  and  generally  also  long  ranks  of 
sporangia  in  all  stages  of  development.  In  some  sporangia  spores 
may  be  found.  Sporangia  and  spores  are  always  readily  got, 
but  care  must  be  taken  to  select  fruit-dots  which  are  not  too  old 
or  too  young. 

Sprouting  the  Spores.  To  obtain  good  specimens  of  s]3rout- 
ing  spores  and  i^rothallm  free  yro?n  dirt^  we  can  recommend  the 
following  procedure :  Fill  several  small  flower -pots,  which  have 
been  thoroughly  cleaned  inside  and  out,  with  clean  fine  sand. 
Sterilize  the  whole  by  baking  in  an  oven  or  a  hot-air  sterilizer. 
Set  the  pots  into  large  (porcelain)  dishes  capable  of  holding  water, 
and  keep  the  bottom  of  these  dishes  covered  to  the  de]3th  of  one 
inch  with  water;  cover  the  pots  completely  ^\itli  bell-glasses. 
After  twenty-four  hours,  or  after  the  sand  and  the  pots  have  be- 
come thoroughly  wet,  inside  and  outside.^  dust  thickly  the  sand 
and  the  outsides  of  the  pots  T\dth  spores  (obtained  from  fern- 
houses  by  shaking  fertile  fronds  over  white  paper).  Care  must 
be  taken  to  get  spores^  and  not  merely  empty  sporangia.  After  a 
Aveek  or  longer  (sometimes  several  weeks)  a  bit  of  the  surface- 
layer  of  sand  is  removed  to  a  drop  of  water  on  a  slide  and  exam- 
ined for  sprouting  spores.  These  will  often  be  found  in  various 
stages  of  development.     After  a  month  or  two  prothallia  will  ap- 


LABORATORY  STUDIES  AND  DEMONSTRATIONS.        2] 5 

pear  on  the  outside  of  the  pots;  and  as  tliese  are  clean,  tlicy  may 
be  removed  and  examined  (bottom  side  upwards)  free  of  all 
dirt. 

Failing  these,  prothallia  may  almost  always  be  found  in  fern- 
houses  on  the  tops  or  sides  of  the  pots,  and  especially  on  the 
moist  earth  under  the  benches.  Care  should  be  taken  not  to 
confound  prothallia  with  the  lighter  green  and  relatively  coarse 
liverwort  {Lunularicb)  often  found  in  hotliouses. 

The  Sexual  Organs  "of  Prothallia.  AVith  good  clean  speci- 
mens these  are  easily  found  with  a  rather  low  power.  Iliglier 
powers  are  needed  to  make  out  details.  If  the  archegonia  and 
and  antheridia  are  young  they  are  green ;  if  old,  brown.  On 
young  prothallia  antheridia  only  are  often  found,  and  on  very 
old  ones  archegonia  only. 

Fertilization.  This  is  not  easy  to  observe,  but  the  attempt 
may  be  made  by  examining  successively  a  number  of  very  fresh 
and  vigorous  prothallia  in  different  stages.  They  must  be 
mounted  carefully  (not  flooded  with  water),  and  spermatozoids 
are  generally  more  easily  found  swimming  about  after  the  speci- 
men has  been  mounted  a  little  while. 

Embryology.  Except  in  its  general  features,  this  is  too  dif- 
ficult for  the  beginner.  He  may,  however,  obsei've  the  later 
stages  by  studying  old  prothallia  with  the  young  fern  just  ap- 
pearing, and  young  ferns  with  the  old  prothallia  still  adherent. 

Chlorophyll  and  Starch.  Vigorous  prothallia  afl:"ord  excellent 
examples  of  cells  bearing  chlorophyll-bodies  in  which  starch  is 
easily  detected.  Some  of  the  marginal  cells  should  be  examined 
with  the  highest  power,  attention  being  given  to  the  chloro- 
phyll-bodies and  their  arrangement.  In  favorable  cases  one  may 
observe  the  opaque  rod-like  or  oval  grains  inside  the  latter, 
and  prove  by  reagents  that  they  are  starch  grains. 

The  student  should  also  examine,  at  this  point,  tlie  large 
chromatophores  of  Nitella^  which  may  be  obtained  ]>y  pressing 
out  a  drop  of  the  contents  from  an  internodal  cell,  adding  dilute 
iodine  solution,  and  examining  with  a  high  })ower.  In  favor- 
able cases  as  many  as  a  dozen  starch  grains,  stained  blue,  may  bo 
found  inside  a  single  elliptical  chlorophyll-body. 


216  APPENDIX. 

Chapter  XII.      (Amceba.) 

Amoeba  is  one  of  the  most  capricious  of  animals,  appearing 
and  disappearing  with  inexpHcable  suddenness,  and  as  a  rule  it 
cannot  be  found  at  the  time  when  needed ,  unless  special  prepara- 
tions hav^e  been  made  in  advance.     It  is  never  safe  to  trust  to 
chance  for  a  supply  of  material.      It  is  equally  unsafe  to  trust  to 
the  methods  usually  prescribed.     Amoebae  may,  however,  often 
be  procured  in  abundance  and  with  tolerable  certainty  as  follows  : 
A  month  or  six  weeks  beforehand  collect  considerable  quantities 
of  water-plants  (especially  Nitella  or  Chara)  from  various  pools 
or  slow  ditches,  with  an  abundance  of  sediment  from  the  bottom. 
It  is  important  to  select  clear,  quiet  pools  containing  an  abun- 
dance of  organic  matter  (such  as  desmids,  diatoms,  etc.,  in  the 
sediment) — not  temporary  rain-pools  or  such  as  are  choked  with 
inorganic  mud  (dirt  washed  in  by  rain).      The  material  thus  pro- 
cured should  be  distributed  in  numerous  (10  to  20)  open  shallow 
dishes  (earthenware  milk-pans)  and  allowed  to  stand  about  the 
laboratory  in  various  places — some  exposed  to  the  sun,  others  in 
the  shade.      The   contents  of  many,  perhaps  all,  of  the  vessels 
will  undergo  putrefactive  changes  and  swarm  with  life — first  with 
bacteria,  later  with  infusoria — and  will  then  gradually  become 
clear  again  as  in  a  hay-infusion.     The  sediment  should  now  be 
examined  at  intervals,  and  Amoebce  are  almost  certain  to  appear, 
sooner  or  later,  in  one  or  more  of  the  vessels.      Usually  the  small 
A.  radiosa  appears  first,  but  these  should  only  be  used  if  it  is 
found  impossible  to  procured.  Proteus.,  which  is  far  larger,  clearer, 
and   more  interesting.        Experience  will   show   that  particular 
pools  always   yield  a  crop  of  Amcehm^    while    others    do  not. 
When  once  a  productive  source  is  found  all  trouble  is  ended. 

If  possible  a  sediment  should  be  selected  that  swarms  with 
AmoebcB.  It  is  very  discouraging  for  students  to  pass  most  of 
their  time  looking  for  the  animals  instead  of  at  them.  Large 
cover-glasses  should  be  used,  and  the  material  taken  with  a 
pipette  from  the  very  surface  of  the  sediment  (not  from  its 
deeper  layers).  When  first  mounted  the  animals  are  usually  con- 
tracted, and  only  become  fully  extended  after  a  time.  Outline 
sketches  should  be  made  at  stated  intervals,  the  structure  of  the 
protoplasm   carefully    studied,      the    pulse    of    the    contractile 


LABORATORY  STUDIES  AND   DEMONSTRATIONS.        217 

vacuole  timed  (vary  by  vanning  temperature),  and  the  effect  of 
tapping  the  cover-glass  noted.  It  is  practically  useless  to  look 
for  iission,  for  encysted  forms,  or  for  the  external  opening  of  the 
contractile  vacuole;  and  the  ingulhng  of  food  or  passing  out 
of  waste  matters  is  rarely  seen.  The  formation  of  pseiid(»i)odia 
should  he  carefully  studied.  After  exanunhig  the  living  aTiimals 
they  should  be  killed  and  stained  with  dilute  iodine. 

Arcella  is  almost  always,  and  Diffliujia  sometimes,  found 
Avith  Arnceha.      These  forms  may  be  examined  for  comparison. 

It  is  desirable  also  to  compare  white  l)lood-corpuscles,  which 
may  be  obtained  either  by  pricking  the  finger  or,  better,  from  a 
frog  or  newt.  A  drop  of  blood,  received  upon  a  slightly  warmed 
slide,  should  be  covered  and  sealed  with  oil  aroimd  the  edtre  of 
the  cover-glass.  The  white  corpuscles  are  at  first  rounded,  l)ut 
soon  begin  to  show  change  of  form.  (Xo  contractile  vacuole,  no 
differentiation  into  ectoplasm  and  entoplasm,  often  no  nucleus 
visible.) 

Chapter  XII.    (Infusoria.) 

Parammcia  are  almost  certain  to  ap2:>ear  in  the  earlier  stages 
of  the  ATYioeha  cultures,  and  in  similar  decomposing  licpiids  or 
infusions,  and  to  ensure  having  them  a  large  number  of  vessels 
and  jars  containing  an  excess  of  vegetable  matter  should  be  i)re- 
23ared  a  month  or  more  beforehand.  Their  successful  study  is 
very  easy  if  they  are  procured  in  "cery  large  miirLbers  (the  water 
should  be  milky  with  them),  otherwise  it  is  practically  im]")ossible. 
Three  slides  of  them  should  be  pre2)ared  and  set  aside  for  a  short 
time  (under  cover,  preferably,  in  a  moist  chamber)  to  allow  the 
animals  to  become  quiet.  One  slide  should  contain  simply  a 
drop  of  the  infusorial  water ;  a  second  the  same,  with  the  addi- 
tion of  a  little  powdered  carmine;  to  the  third  add  a  (h*oj)  or  two 
of  an  aqueous  solution  of  chloral  hydrate  (made  by  droi)i)ing  a 
crvstal  or  two  into  a  watch-o-lass  of  water).  The  tirst  slide 
should  be  studied  iirst;  and  it  will  usually  be  fiamd  that  after  a 
time  the  animals  crowd  about  the  edges  of  the  cover,  often  lying 
nearly  or  quite  still.  If  this  is  not  the  case,  the  sj)ecimens  j)ara- 
lyzed  by  chloral  may  be  studied.  The  carmine  sjiecimens  will 
show  beautiful  food-vacuoles  tilled  witli  carmine;  and  bv  careful 
study  the  formation  of  the  vacuoles  may  be  observed. 


218  APPENDIX. 

The  general  strncture  sliould  be  carefully  studied,  the  con- 
tractile vacuoles  particularly  examined  (they  are  seen  best  in  dying 
specimens  or  in  those  paralyzed  by  chloral),  and  dividing  or  con- 
jugating individuals  looked  for  (they  are  often  abimdant).  The 
only  really  difficult  point  is  the  nucleus,  which  cannot  be  well 
seen  in  the  living  animal.  It  may  be  clearly  seen  by  mounting 
a  drop,  to  which  a  little  dihite  iodine  or  2^  acetic  acid  has  been 
added.  The  former  shows  the  cilia  well,  the  latter  the  tricho- 
cysts.  Osmic  acid  and  corrosive  sublimate  also  give  good  preser- 
vation. The  internal  changes  during  fission  and  conjugation 
must  be  studied  in  prepared  specimens  mounted  in  balsam.  Such 
preparations  are  often  of  great  beauty  and  interest. 

Vorticella  must  be  sought  for  on  duck-weed  or  other  plants, 
or  on  floating  sticks,  and  the  like.  Zodthamnion^  Carchesium, 
etc. ,  are  liable  to  appear  at  any  time  in  the  aquaria.  All  these 
forms  are  easily  studied.  Conjugation  is  very  rarely  seen,  but 
fission  and  motile  forms  are  common.  The  macronucleus  is 
especially  well  shown  in  dead  or  dying  specimens. 


Chapter  XI Y.   (Pkotococcus.) 

Protococcus  {PleurocoGGus)  is  found  in  abundance  on  the 
northerly  side  of  old  trees  in  many  parts  of  the  United  States. 
In  case  it  cannot  be  obtained  in  any  region  it  may  be  procured, 
during  1895  and  1896,  from  Prof.  Sedgwick,  Institute  of 
Technology,  Boston,  Mass. ,  by  mail.  The  laboratory- work  with 
it  is  too  easy  to  require  comment.  See,  however,  Arthur, 
Barnes  &  Coulter's  "Plant  Dissection"  (Henry  Holt  &  Co., 
ISTew  York). 

Chapter  XY.     (Yeast.) 

Bakers',  brewers',  compressed,  and  dried  yeast  may  be  had 
in  the  markets.  Brewers'  yeast  is  to  be  preferred,  as  com- 
pressed yeast-cakes  contain  starch,  bacteria,  and  other  extraneous 
matters.  All  of  the  kinds  may  be  cultivated  to  good  advantage 
in  wort  (to  be  obtained  at  breweries)  or  in  Pasteur's  fluids.  (See 
Huxley  and  Martin,  chapter  on  Yeast.)     Wild  yeasts  may  be 


r 


LABORATORY  STUDIES  AND  DEMONSTRATIONS.        219 

found  by  examining  sweet  cider  niicroscoi)ically.      For  the  fol- 
lowing methods  of  demonstrating  nuclei  in  yeast  and  ohtannn* 
ascospores  we  are  indebted  to  Mr.  8.  C.  Keith,  .Ii-. 

To  Demonstrate  Nuclei  in  Yeast.  Any  <;«)«jd  activelv-irrowint'' 
yeast  will  answer,  but  a  large  (brewers')  yeast  is  preferaljlc.  .Mix 
a  little  of  the  yeast  with  an  equal  amount  of  tap-water  in  a  test- 
tube  and  shake  thoroughly.  Add  an  equal  volnme  of  llermanirs 
fluid  and  shake  again.  As  soon  as  the  yeast  has  settled  j)our  off 
the  supernatant  liquid  and  wash  the  yeast  by  decantation.  Trans- 
fer some  of  the  cells  to  a  slide,  flx  by  drying,  stain  by  Ih'ick'ii- 
hain's  iron-hsematoxylin  method  (see  Centralhlatt  fi'ir  Barirr't- 
ologie^  xiv.  (1893),  pp.  358-300),  wash,  dehydrate  with  alcuhul, 
follow  with  cedar-oil,  and  mount  in  balsam.  In  successful  speci- 
mens the  effect  is  very  satisfactory.      (See  Fig.  96.) 

A  Simpler  Method.  To  demonstrate  nuclei  in  yeast  more 
quickly  and  very  easily  the  following  method  may  be  used  :  Pxjil 
(in  a  test-tube)  for  a  moment  an  infusion  of  vey^y  vigorous  yeast 
in  water,  place  a  drop  of  the  boiled  infusion  on  a  slide,  add  a 
drop  of 'y<?/'y  dilute  "Dahlia"  solution,  cover,  and  after  one  or 
two  minutes  examine  with  a  high  power.  The  nuclei  in  most  of 
the  cells  will  be  easily  discoverable. 

To  Obtain  Ascospores  in  Yeast.  It  has  been  usually  recom- 
mended to  employ  for  this  purpose  blocks  of  plaster-of-Paris. 
"We  have  found  the  following  method  more  trustworthy  : 

The  yeast  to  be  used  should  be  the  "  top"  yeast  used  in  ale- 
breweries.  It  should  also  be  actively  growing  and  fresh.  If 
fresh  yeast  cannot  be  obtained,  some  may  be  revived  by  culti\a- 
tion  for  2-1:  hours  at  25°  C.  in  wort,  and  a  little  of  the  thick  sedi- 
mentary portion  may  then  be  placed  in  a  very  thin  layer  on  dry 
filter-paper  which  has  previously  been  sterilized  by  baking.  The 
filter-paper  is  then  j^laced  on  a  layer  of  cotton  about  \  incli  in 
thickness  lying  on  a  plate  or  saucer,  the  cotton  having  })revit»usly 
been  thoroughly  wetted  with  cold  sterilized  tap-watiM-,  The 
whole  is  covered  by  a  bell-glass  and  set  in  a  ratiier  warm  place 
(25°  C).  In  the  course  of  two  or  three  days  spores  will  be  found 
in  many  of  the  cells.  The  lower  the  tenq)erature  the  longer  ia 
the  time  required  for  spore  formation.  If  "bottom"  yeast  is 
used  instead  of  "top"  yeast  a  nnich  longer  time  is  required,  and 
the  results  are  far  more  uncertain. 


220  APPENDIX. 


Chapter  XYI.     (Bacteria.) 

For  the  study  of  Bacteria  it  is  very  desirable  to  have  a  largo 
species,  and  for  this  purpose  there  is  none  better  than  Bacillus 
megaterium,  which  may  be  obtained  from  almost  any  bacteriologi- 
cal laboratory  and  grown  in  the  bouillon  used  by  bacteriologists. 
During  1895  and  1896  it  may  be  obtained  from  Boston  (see 
above).  Tliis  form  is  very  large,  and  produces  spores  readily. 
(See  De  Bary,  "  Lectures  on  Bacteria;"  Sternberg,  ''Bacteriol- 
ogy;" Abbott,  "Principles  of  Bacteriology;  "  etc.)  The  pro- 
longed study  of  bacteria  is  not  suited  to  beginners.  Vinegar 
bacteria  may  be  seen  in  the  mother- of -vinegar  by  pressing  a  bit 
of  it  out  under  a  cover  slip  and  examining  with  a  high  power. 
The  jelly  of  mother-of- vinegar  is  a  good  example  of  zooglma. 
The  white  scum  which  apj^ears  on  aquaria  and  infusions  is  of 
the  same  general  character  (zooglma'). 


Chapter  XYII.     (A  Hay  Infusion.) 

To  make  a  successful  hay  infusion  care  should  be  taken  to 
nse  water  containing  numerous  and  various  organisms,  and  tliere- 
fore  distilled  v^rater,  spring-waters,  and  well-waters,  are  in  general 
to  be  avoided.  Tap- water  should  also  be  avoided  if  it  is  derived 
from  springs  or  wells.  The  l)est  water  for  the  purpose  is  that 
drawn  from  ponds,  rivers,  lakes,  or  other  surface  sources. 
Clean  ditch  or  pool  water  is  excellent.  The  choice  of  hay  is  less 
important,  but  it  is  well  to  avoid  old  hay  and  hay  that  is  very 
woody.  The  infusion  should  be  warmed,  but  not  lieated  or 
boiled.  It  may  be  kept  in  a  beaker  in  diffuse  daylight,  e.g.,  in 
a  north  window,  the  beaker  being  loosely  covered. 

mSTKU]\IENTS  AND  UTENSILS.* 

The  student  should  liave  access  to  the  following  articles : 
A  compound  microscope  with  tvro  eyepieces  and  low  and 
high  power  objectives  (i.e.,  about  1  in.  and  \  in.,  or  objectives 

*  Most  of  the  apparatus  and  reagents  here  mentioned  may  be  obtained  from 
any  first-class  dealer  in  physical  and  microscopical  apparatus,  e.g.,  from  The 


LABORATORY  STUDIES  AND  DEMONSTRATIONS.        221 

A  and  D  of  Zeiss,  or  J  and  ^  incli  of  Bauscli  and  Lonib ;  still 
higher  powers  are  desirable). 

A  simple  dissecting  microscope;  a  desirable  f(jrm  is  an  ordi- 
nary watchmaker's  lens  provided  with  a  support.  An  (>r(lin;iry 
pocket-lens;  glass  slides  (3  X  lin.),  cover-glasses,  watch-crystals, 
small  gummed  labels,  needles  with  adjustable  handles,  camel' s- 
hair  brushes,  blotting  and  lilter  paper,  a  good  razor,  pipettes 
(medicine-droppers),  glass  rods  and  tubes,  glass  or  pon-elain 
dishes  for  staining,  etc.,  a  set  of  small  dissecting  instruments 
(small  scalpel,  forceps,  and  straight-pointed  scissors),  a  section- 
lifter,  pieces  of  pith  for  section-cutting,  thread,  a  shallcnv  tin  pan 
lined  with  wax,  long  insect  pins  for  pinning  out  dissected  s])eci- 
mens,  drawing  materials,  and  a  note-book  for  sketches  and  other 
records. 

Each  table  should  be  furnished  with  a  set  of  small  reafjent- 
bottles,  a  Bunsen  burner,  wash-bottle,  test-tubes,  beakers,  and  a 
bell-glass  for  protection  from  dust.  Thermometers,  a  balance, 
microtome,  drying  oven,  and  a  paraffin  water-bath  should  also  be 
accessible. 

REAGENTS  AND  TECHNICAL  METHODS.* 

Alcohol. — Since  biological  laboratories  belonging  to  incorpo- 
rated institutions  obtain  alcohol  duty  free,  it  should  be  UheniUy 
supplied  and  freely  used.  Alcohol  of  100°,  i.e.,  "absolute" 
alcohol,  may  be  purchased  in  1-pound  bottles.  "Squibb's" 
absolute  alcohol  may  be  obtained  of  any  druggist, f  but  ordinary 
alcohol  of  90-95^  answers  nearly  every  purpose.  ''Cologne 
spirits,"  i.e.,  alcohol  of  about  94^,  may  be  obtained  from  the 
distillers  at  60c.,  or  thereabouts,  per  gallon.      It  may  then  be 

Bausch  &  Loinb  Optical  Co.,  Rochester,  N.  Y.;  the  Fnmkliu  Educational 
Co.,  Hamilton  Place,  Boston;  or  Queen  &  Co.,  Chestnut  Street,  Phihulelphia. 
Chemical  and  other  apparatus  may  be  obtained  from  Eimer  &  Amend,  205-211 
Third  Avenue,  N.  Y. 

*  Every  laboratory  should  be  supplied  with  some  of  the  standard  books  upon 
this  subject,  e.g.,  Strasburger's  Botanisclie  Practicinn,  Jena;  Whitniun'a 
Methods  of  Research  in  Microscopical  Anatomy  and  Embryology,  Boston:  Lee, 
The  Microtomist's  Vade  Mecum,  last  edition;  Zimmerman's  Botanical  Micro- 
technique (Humphrey),  Holt,  N.  Y. 

f  See  also  Whitman,  1.  c,  p.  14. 


222  APPENDIX. 

diluted  to  80^,  70^,  50^,  etc.,  as  needed.     For  tins  purpose  an 
alcoliolinieter  is  very  convenient. 

Acetic  Acid. — One  or  two  parts  glacial  acetic  acid  to  100  parts 
water. 

Acetic  Acid  and  Methyl-green. — This  is  valuable  for  staining 
nuclei  in  vegetal  tissues.  Dissolve  metlijl-green  in  one  or  two 
per  cent  acetic  acid  until  a  rich,  deep  color  is  obtained. 

Borax-carmine. — Add  to  a  4^  aqueous  solution  of  borax  2-3^ 
carmine,  and  heat  until  the  carmine  dissolves.  Add  an  equal 
volume  of  70^  alcohol,  and  filter  after  24  hours.  After  staining 
(6-12  hours,  or  more  for  large  objects,  a  few  minutes  for  sec- 
tions) place  the  object  in  acidulated  alcohol  (100  c.c.  35^  alcohol, 
3-4  droj)s  hydrochloi'ic  acid)  and  leave  until  the  color  turns  from 
dull  to  bright  red  (10-30  m.).  Afterwards  remove  to  70^ 
alcohol. 

Canada  Balsam,  Mounting  in. — This  invaluable  substance  may 
be  obtained  in  the  crude  condition,  dried  by  prolonged  heating, 
and  then  dissolved  in  chloroform,  benzole,  or  turpentine,  for 
use.  The  benzole  solution  is  perhaps  the  best,  and  may  be  ob- 
tained from  most  of  the  dealers.  The  principles  of  mounting  in 
balsam  are  veiy  simple.  It  does  not  mix  with  water  or  alcohol, 
but  mixes  freely  with  clove-oil,  chloroform,  benzole,  etc.  Ob- 
jects are  therefore  generally  treated,  first  with  very  strong  alco- 
hol, 95-100^,  in  order  to  remove  the  water ;  then  with  clove-oil, 
chloroform,  or  turpentine  to  remove  the  alcohol,  and  afterwards 
mounted  in  a  drop  of  balsam.  This  should  usually  be  placed  on 
the  cover-glass,  which  is  thereujDon  inverted  over  the  object. 
The  balsam  gradually  sets  and  the  preparations  are  permanently 
preserved. 

Carmine. — Carmine  may  be  obtained  as  a  powder,  which 
when  rubbed  up  thoroughly  with  water  in  a  mortar  passes  into  a 
state  of  very  fine  subdivision.  .  This  proj)erty  makes  it  available 
for  experiments  with  cilia,  etc. 

It  is  more  often  used  in  solution,  as  a  staining  agent.  (See 
Borax- carmine.) 

Cellulose- test. — Saturate  the  object  in  iodine  solution,  wash  in 
water,  and  j^lace  it  in  strong  sulphuric  acid  j)repared  by  carefully 
pouring  2  volumes  of  the  concentrated  acid  into  1  volume  of 
water. 


LABORATORY  STUDIES  AND  DEMONSTRATIONS.        22^ 

Collodion  and  Clove-oil. — Used  for  fixiiiij:  sections  to  the  slide 
in  order  to  prevent  the  displacement  of  deUcate  or  isolated  part& 
in  balsam-monnting.  Mix  one  part  of  ether-coll()di<»n  and  tliree 
parts  of  oil  of  cloves.  In  niountiiii^,  varnish  a  sHde  with  the 
mixture  by  means  of  a  camel' s-hair  brush,  lay  on  tlie  sections, 
and  place  the  slide  for  a  few  minutes  on  the  water-hath  (i.e., 
until  tlie  clove-oil  evaporates).  Transfer  the  slide  trj  a  wide- 
mouthed  bottle  of  turpentine  (to  dissolve  tlie  ])arathn),  remove  it 
and  drain  off  the  turpentine,  place  a  drop  of  Canada  balsam  on 
the  middle  of  a  cover-glass,  and  invert  it  over  the  object. 

Dahlia. — Dissolve  in  water. 

Eosin. — Dissolve  in  water  until  a  bright-red  solution  is  ob- 
tained.     It  should  be  diluted  when  used. 

Glycerine,  dilute. — Two  parts  glycerine,  one  part  distilled 
water. 

Haematoxylin  (Delafield's). — Add  4  c.c.  of  saturated  alcoholie 
solution  of  hgematoxylin  to  150  c.c.  of  strong  aqueous  solution  of 
anmionia-alum ;  let  the  mixture  stand  a  week  or  more  in  the 
light,  filter,  and  add  25  c.c.  of  glycerine  and  25  c.c.  of  methyl 
alcohol.  The  fluid  improves  greatly  after  standing  some  weeks 
or  months. 

Haematoxylin  (Kleinenberg^s). — To  a  saturated  solution  of  cal- 
cium chloride  in  70^  alcohol  add  an  excess  of  pure  alum ;  filter 
after  24  hours  and  add  8  volumes  of  70^  alcohol,  filtering  again 
if  necessarv.  Add  a  saturated  alcoholic  solution  of  lueniatoxylin 
until  the  liquid  becomes  purple- blue.  The  longer  the  liquid 
stands  before  using,  the  better.  It  should  be  diluted  for  use 
with  the  alum-calcium-chloride  solution  in  70,^^  alcohol. 

Hermann's  Fluid. — See  Lee's  Vade  Mecum. 

Iodine  Solution. — Dissolve  jiotassium  iodide  in  a  small  quantity 
of  water,  add  metallic  iodine  until  the  mixture  assumes  a  dark- 
brown  color,  and  then  dilute  to  a  dark-sherry  color.  The  solu- 
tion should  be  kept  from  the  light. 

Magenta  (Aniline  Red). — Dissolve  in  water. 

Methyl  Green. — Used   in   aqueous   or  alcoholic    solution    or 

with  acetic  acid. 

Normal  Fluid  (Normal  Salt  Solution). — Dissolve  7.50  grams  of 
sodium  chloride  in  1  litre  of  distilled  water. 

Paraffin.— "Hard"  and  "soft"  paraffins,  i.e.,  those  of  high 


224  APPENDIX. 

and  low  melting-points,  should  be  mixed  in  such  proportions  that 
the  melting-point  lies  between  50°  and  55°  C. 

Perenyi's  Fluid, — Ten-per-cent  nitric  acid  4  parts,  90^  alco- 
hol 3  parts,  ^fc  aqueous  solution  of  chromic  acid  3  parts.  Not 
to  be  used  until  the  mixture  assumes  a  violet  hue.  Leave  objects 
in  the  fluid  30  minutes  to  an  hour,  then  24  hours  in  70^  alcohol, 
and  finally  place  in  90  per  cent  alcohol. 

Schultze's  Macerating  Fluid. — Dissolve  a  gram  of  potassium 
43lilorate  in  50  c.c.  of  nitric  acid.  The  tissue  should  be  boiled 
in  the  mixture  and  afterwards  thoroughly  washed  in  water. 

Schulze's  Solution. — Dissolve  zinc  in  pure  hydrochloric  acid, 
evaporate  in  the  presence  of  metallic  zinc,  on  a  water-bath,  to  a 
syrupy  consistency,  add  as  much  iodide  of  potassium  as  will  dis- 
solve, and  then  saturate  Avith  iodine.  (When  heated  with  this 
fluid  cellulose  turns  blue. 

Section-cutting. — Many  objects  can  be  cut  by  hand  with  a 
razor  (which  must  be  very  sharp).  The  object  should  be  held  in 
the  left  hand  while  the  razor  is  pointed  away  from  the  body,  and 
allowed  to  rest  on  the  tips  of  the  fingers  with  its  edge  turned 
towards  the  left.  It  is  then  drawn  gently  toAvards  the  body  so 
as  gradually  to  shave  oft*  the  section.  Small  objects  may  be  held 
between  two  pieces  of  watchmaker's  pith  previously  soaked  in 
water.      In  either  case  the  razor  should  be  kept  wet. 

Many  objects,  however,  require  more  careful  treatment  by 
one  of  the  following  methods  : 

A.  Paraffin  Method. — After  hardening  and  staining,  the 
object  is  soaked  in  strong  alcohol  (95^  or  more)  until  the  Avater 
is  thoroughly  extracted  (2-12  hours,  changing  the  alcohol  at 
least  once),  then  in  chloroform  until  the  alcohol  is  extracted 
(2-12)  hours),  and  then  in  melted  parafiin  (not  warmer  than  55° 
C.)  on  a  water-bath  for  15  to  30  minutes  (too  high  a  tem^^era- 
ture  or  too  long  a  bath  causes  excessive  shrinkage).  Some  of  the 
parafiin  is  then  poured  into  a  small  paper-box,  or  into  adjustable 
metal  frames.  The  object  is  transferred  to  it  and  after  the  mass 
has  begun  to  set  it  is  placed  in  cold  Avater  until  quite  hard.  It 
is  then  cemented  (by  paraffin)  to  a  square  piece  of  cork  and 
placed  in  the  section-cutter  or  microtome. 

The  sections  may  be  cut  singly  with  the  oblique  knife  or  by 


LABORATORY  STUDIES  AND  DEMOXSTRATIOSS.        2*25 

the  ribbon- method,'^  the  knife  being  "kept  dry  in  eitlier  cai^e.  In 
mounting  they  should  be  lixed  by  tlie  culludiun-niethod.  (^Seo 
Collodion  and  Clove-oil.) 

B.  Celloidln  MdluxJ. — This  is  especially  ap])licable  to  <k'li- 
cate  ve<>:etal  tissues.  After  deli vd rat iu*^  the  obifc-t  thomuirldv  in 
alcohol,  soak  it  24  hours  in  a  mixture  of  ecpial  parts  of  alcohol 
and  ether.  Make  a  thick  solution  of  celloidin  in  the  same  mix* 
ture  and  soak  the  object  for  some  hours  in  it.  It  may  then  ho 
imbedded  as  follows:  Dip  the  smaller  end  of  a  tapering  (M»rk 
in  the  celloidin  solution,  allow  it  to  dry  for  a  monieiit  (hh»wing 
on  it  if  necessary),  and  then  build  upon  it  a  mass  of  celloidin, 
allowing  it  to  dry  a  moment  after  each  additi<»n.  Transfer  the 
object  to  the  cork  and  cover  it  thoroughly  with  the  celloidin. 
Then  float  the  cork  in  82-85^  (0.842  sp.  gr.)  alcohol  until  the 
mass  has  a  firm  consistency  (24  h.).  It  may  then  be  cut  in  the 
microtome  with  the  oblique  knife,  which  nnist  be  kept  dripping 
with  82-85,^  alcohol.  Keep  the  sections  in  82-85^  alcohol  until 
ready  to  mount  them,  then  soak  them  for  a  minute  in  strong 
alcohol,  transfer  to  a  slide,  pour  on  chloroform  until  the  alcohol 
is  removed,  drain  off  the  liquid,  quickly  add  a  drop  of  balsam, 
and  cover.     (See  also  Whitman,  1.  c,  p.  113.) 

*  See  Whitman,  1.  c.  p.  71. 


INDEX. 


Absorption,  48,  52,  101,  165c 
Accretion,  166, 
Acbromatin,  23. 
Actinopbrys,  166. 
Adaptation,  97,  98,  144. 
Adventitious  buds,  130. 
Probes,  202. 
Etiology,  6. 

Agamogenesis,  73,  130,  163. 
Albuminous  bodies,  36. 
Alimentation,  48,  105. 
Alimentary  canal,  82,  92. 
Alimentary  system,  49. 
Allolobophora,  41. 
Alternation  of  generations,  130, 
Ammha,  27,  158,  216. 
Amoeboid  cells,  64. 
Ampliiaster,  84. 
Amphimixis,  168. 
Anabolism,  33,  100,  149,  164 
Anachai'is,  29. 
Anaerobes,  202. 
Anatomy,  7. 
Animalcule,  158,  199. 
Annulus,  132. 
Anus,  46,  82,  165. 
Antberidia,  135. 
Aortic  arcbes,  54,  55. 
Apical  buds.  111,  116,  123. 
Apical  cell,  123. 
Apogamy,  143. 
Apospory,  143. 
Areella,  166. 
Archegouia,  137. 
Arcbenteron,  80,  82,  85. 
Arcbesporium,  131. 
Arcboplasm,  79,  80. 
Artbrospore,  195. 
Ascospore,  187. 
Asexual  reproduction,  73. 
Assimilation,  182. 
Aster,  79,  84. 
Attraction  spbere,  83,  84. 
At  WATER,  W.  O.,  34^. 

Bacilli,  192. 
Bacteria,  64,  178,  192. 
Bast-fibres,  120. 


Biology,  1,  6,  7.  8. 
Bisexual,  73,  l.'W. 
Blastoi)()re,  80,  b5. 
Blastospbere,  85. 
Blastula,  80,  90. 
Blood.  15,  16,  90,  102. 
Blood-vessels,  54. 
Blue-green  alg;e,  183,  193o 
Body,  19,  24.  84,  107,  156. 
Body-cavity,  47. 
Bone,  16. 
Botan\,  6,  7. 
Branches,  111,  122,  130. 
Brancbijp,  62. 
Budding,  186. 
Biirsaria,  176. 

Calciferous  glands,  51. 
Calkins,  G.  X.,  171. 
Capillaries,  54. 
Capsules  of  eggs,  78. 
Capsulogenous  glands.  46. 
Carbohydrates,  37,  101. 
CairJu'sium,  176. 
Carnivora.  177,  203. 
Cartilage,  15,  1(5. 
Castings,  42,  53. 
Cell,  12,  20. 
Cell-division,  24,  83. 
Cell-theorv.  20. 
Cellulose,  "37. 
Cell-wall,  22,  23. 
Centrosome,  79,  83,  84. 
Cerebral  ganglia,  65,  69. 
Chalk,  166, 
Chara,  24. 
Chemiotaxis,  139. 
Chlorococcus,  178. 
Cliloragogiie-cclls,  52,  61,  93. 
Cbloropbvll,  12<).  151.  215. 
Chlorophyll -bodies.  179,  215. 
Vhroococcuti,  183. 
Chromatin,  23,  83. 
Cliroinutoi)hores.  147,  179, 
Chromosomes.  83.  84. 
Cilia.  31.  63,  74,  137.  192. 
Circulation.  4S,  53,  101,  165. 
C'LArAliiDE,  96. 


228 


INDEX. 


Classification,  7. 
Clitellum,  46,  77,  78,  88,  93. 
Coagulation,  36,  39. 
Cocci,  192. 
Coelenterata,  88. 
Coelom,  47,  82. 
Coelomic  Huid,  53. 
COHN,  21. 
Cold  storage,  199. 
Colloidal,  36. 
Colony,  176. 
Commissures,  65. 
Conjugation,  171,  181. 
Connective  tissue,  70,  90. 
Consciousness,  69,  70. 
Contractility,  62,  164. 
Coordination,  48,  64,  67,  164. 
Copulation,  77. 
Cross-fertilization,  74. 
Crystals,  17. 
Cushion,  135. 
Cuticle,  71,  91. 
Cyanopbyceae,  183,  192,  199. 
Cyclical  change,  5,  72,  89. 
Cytoplasm,  22,  84. 

Darwin,  42,  51,  70,  99,  103. 

Death,  152. 

De  Bary,  115,  143. 

Defsecation,  53,  165. 

Desmids,  178,  183. 

Dialysis,  36,  210. 

Diastatic  ferment,  52. 

Diatoms,  178,  183. 

Dichogamy.  138. 

Differentiation,  11,  84,  141. 

Differentiation,    antero-posterior,    43, 

110. 
Differentiation,  dorso-ventral,  43,  110. 
Differentiation  of  the  tissues,  25. 
Diffliigia,  166. 

Digestion,  48,  49,  52,  101,  165. 
Diplococcus,  194. 
Disease-germs,  192,  197. 
Disinfection,  200. 
Dissepiments,  47,  94. 
Distribution,  7. 

Division  of  labor,  11,  26,  156,  165. 
Dorsal  pore,  48. 
Dorsal  vessel,  54. 

DUJARDIN,  21. 

Earthworm,  41. 
Ectoblast,  81. 
Ectoplasm,  158. 
Egg,  24. 
Egg  laymg,  77. 
Egg-nucleus,  79. 
Egg-string,  74. 
Embryo,  25. 
Embryology,  7,  72,  78. 


Endospore,  187,  194. 

Endosporium,  134. 

Energy,  32,  99,  146,  151. 

Entoblast,  81. 

Entoplasm,  158. 

Environment,  97,  103,  144,  15t 

Epidermal  system,  114. 

Epidermis,  114,  116. 

Epistylis,  176. 

Epithelium,  90. 

Eagle  na,  176. 

Excretion,  48,  53,  59,  100,  165. 

Exosporium.  134. 

Eve-spot,  176. 

Faeces,  53. 

Farlow,  143. 

Fats,  17,  37,  101. 

Feathers,  18. 

Ferns,  105. 

Ferment,  52. 

Fermentation,  191,  197. 

Fertilization,  73,  78,  139. 

Fibro- vascular  system,  114. 

Fibro- vascular  bundles,  142. 

Filtration,  200. 

Fission,  163. 

Flagellum,  176,  192. 

FOL,  79. 

Foods,  146. 

Foraminifera,  166. 

Fore-gut,  86. 

Foster,  Michael,  153,  163. 

Fredericq,  52. 

Frond,  125. 

Functions,  9. 

Fundamental  system,  114. 

Fungi,  147. 

Gamete,  181. 

Gamogenesis,  73,  130,  168. 
Ganglion,  64,  94. 
Gastrula,  80. 
Gastrulation,  84. 
Germ- cells,  24,  73,  90,  130. 
Germination,  134. 
Germ-layers,  81,  84,  85o 
Germ-layer  theory,  88. 
Germ- plasm,  89,  152. 
Germinal  spot,  74. 
Germinal  vesicle,  74. 
Giant-fibres,  94. 
Gills,  62. 
Girdle,  78. 
Gizzard,  51,  71. 
Glmocapsa,  178,  183. 
Glucose,  52. 
Glycogen,  37. 
Gregarinn,  64. 
Growth,  165. 
Guard-cells,  128. 


INDEX, 


229 


Hdmatococcus,  178. 
Haemoglobin,  54. 
■lair,  18. 

Hay  infusion,  201. 
•ierbivora,  176,  203, 
ileredity,  84. 
Hermaphrodite,  73,  130, 
Heutwig,  79. 
Hibernation,  38. 
Hind-gut,  86. 
Histology,  7. 
HooKE,  Robert,  20. 
H30KER,  Sir  W.  J.,  106 
Hoppe-Seyler,  35. 
Huxley,  2,  4. 
Hypodermis,  92. 

Impregnation,  73,  139. 
Individual,  13,  156,  164. 
Indusium,  131. 
Infusions,  168. 
Infusoria,  168,  217. 
Inheritance,  80,  84. 
Intussusception,  4,  165, 
Irritability,  164. 

Johnson,  35. 

Katabolism,  33,  99,  149,  164. 
Karvokinesis,  83. 
Keith,  S.  C,  Jr. ,  186,  195. 
Krukenberg,  52. 

Lateral  ridges,  111,  114. 
Leaf,  11,  125. 
Lenhossek,  95. 
Leptothrix,  194. 
LlNN^US,  105. 
Lumhricwi,  41. 
Lungs,  62. 
Lymph,  53. 
Lymph-cells,  64. 

Macrogamete,  175. 

Macro  nucleus,  170,  171. 

Malic  acid,  139. 

Maupas,  170. 

Meristem,  123. 

Mesoblast,  81. 

Mesophyll,  126. 

Metabolism,  33,  100,  101,  148.  164. 

Metamerism,  45. 

Metchntkoff,  53. 

Microgamete,  175. 

Micronucleus,  170,  171, 

Micro-organisms,  201. 

Middle-piece,  74,  79,  Sa 

Mid-gut,  86. 

Mitosis,  83. 

MoiiL,  H.  VON,  21. 

Morphology,  6,  7. 


Mother-of- vinegar,  194,  195. 
Mother-cells,  134,  137. 
Motion,  48. 
Motor  system,  62. 
Mouth,  49.  80,  85,  165. 
Muscles,  14,  26.  27,  62,  90. 
MujvDEu,  35. 
Mt/ro(lrrma,  194,  202. 
Myxobacteria,  199. 
Myxomycetes,  199. 

Natural  selection,  99. 

Nei)hridia,  58,  59. 

Nerves,  64,  90. 

Nerve-cells,  94. 

Nerve-centre,  68. 

Nerve-impulses,  67. 

Nervous  system,  64,  82,  94,  103. 

Nitella,  28. 

Nitrogen,  147. 

Nucleolus,  23. 

Nucleoplasm,  22. 

Nucleus,  16,  23,  186. 

Nutrition,  99,  146. 

(Esophagus,  18. 
Old  age.  72,  152,  166. 
Odphore,  130. 
05sphere,  73,  138. 
Oospore,  139. 
Organisms,  9. 
Organogeny,  85. 
Organs,  9. 
Ovaries,  74. 
Oviduct,  75. 
Ovum,  73,  74,  89. 

Prira?ncecium.  168. 

Parasites,  192. 

Parenchyma,  116. 

Pasteur,  188. 

Pasteurization,  200. 

Pasteur's  Huid,  189,  197. 

Pathogenic,  200. 

Pathology,  6,  7. 

Peptic  ferment,  52. 

Pept(me,  52.  101. 

Peristaltic  actions,  51,  54,  55. 

Pfkkfku,  139. 

Phagocytes.  53.  61.  64,  158. 

Pharyngeal  ganglia,  67. 

Pharynx,  49. 

Physiological     properties     of     proto 

plasm,  1(;3.  1M2.  183. 
Physiology,  6.  7.  166. 
Plivsiologv  of  the  nervous  svstem,  67. 
Poiarct'lls,  79. 
Pole-cells,  82. 
Poisons,  39. 
Plasma.  53. 
rieurococcns,  178. 


230 


INDEX. 


Primordial  utricle,  29. 
Proctodaeum,  83,  86. 
Pronucleus,  79. 
Prosencliyma,  116. 
Prostomium,  45. 
Protection,  71. 
Proteids,  3,  33,  52. 
Proteus  animalcule,  27,  158. 
Prothallium,  180,  135,  214. 
Pi'otococcus,  178. 
Protonema,  134. 
Protoplasm,  16,  20,  207,  208. 
Protozoa,  158. 
Pseudopodia;  27,  158* 
Psychology,  7,  8. 
Pulse,  54. 
Putrefaction,  197,  201. 

PURKINJE,  21. 

Radiolaria,  166. 

Keceptacle,  131. 

Receptaculum  ovorum,  'J'5. 

Reflex  action,  67. 

Regeneration,  73. 

Reproduction,  48, 72,  111 ,  130, 152, 165. 

Respiration,  61,  150,  165. 

Retzius,  95. 

Rhizoids,  134. 

Rhizome,  111,  140. 

Rhizopoda,  166. 

Rigor  caloris,  39. 

Rigor  mortis,  209. 

Roots,  122. 

SaccJiaromyceSy  184. 
Sachs,  115. 
Salivary  glands,  51. 
Sap,  14. 

Saproph}i;es,  192. 
Sarcina,  194. 
Schizomycetes,  192, 

SCHLEIDEN.   20. 

ScHULTZE,  Max,  21. 

Schwann,  20. 

Sciences,  biological,  1,  6. 

Sciences,  physical,  1. 

Segmentation,  24,  80. 

Segmentation  cavity,  84,  85. 

Seminal  receptacle,  77. 

Seminal  vesicle,  76. 

Sensation,  48. 

Sense  organs,  42,  69. 

Senses,  42,  69. 

Sensitive  system,  69. 

Setae,  46,  63. 

Setigerous  glands,  63,  77. 

Sexual  reproduction,  73. 

Sieve-tubes,  116. 

Sight,  42,  69,  70. 

Skin,  128. 

Slipper  animalcule,  168. 


Smell,  42,  69. 
Sociology,  7,  8. 
Somatic  cells,  73. 
Somatic  layer,  85. 
Somatopleure,  82,  86. 
Somites,  45. 

Spencer,  Herbert,  3,  99,  146. 
Spermaries,  74,  75. 
Spermatosphere,  77. 
Spermatozoid,  137. 
Spermatozoon,  73,  74 
Sperm-duct,  76. 
Sperm-nucleus,  79. 
Spiderwort,  29. 
Spirilla,  192. 
Splanchnopleure,  82,  86. 
Spontaneous  generation,  33. 
Sporangia,  130. 
Spores,  24,  130,  194, 
Sporophore,  130. 
Staphylococcus,  194. 
Starch,  17,  37,  146. 
Stentor,  176. 
Sterilization,  199. 
Stimulus,  67. 
Stipe,  125. 

Stomach-intestine,  51. 
Stomata,  126,  128. 
Stomodaeum,  82,  86. 
Streptococciis,  194. 
Struggle  for  existence,  203. 
Stylonichia,  170. 
Sugar,  37. 

Sun-animalcule,  166. 
Survival  of  the  fittest,  99. 
Symbiosis,  177. 
Symmetry,  bilateral,  44,  110. 
Symmetry,  serial,  45. 
Sympathetic  system,  67. 

Taste,  42,  69,  70. 
Taxonomy,  7. 
Temperature,  38,  199,  210. 
Testes,  74,  75. 
Tissues,  11,  13. 
Touch.  42,  69,  70. 
Toxicologv,  39. 
Trachea?,  116. 
Tracheids,  116. 
Trade scantia,  29. 
Transpiration,  146. 
Trichocysts,  168. 
Tryptic  ferment,  52. 
Twins,  88. 
Typhlosole,  51,  91. 

Unicellular  animals,  158. 
Unicellular  organisms,  156,  177. 
Unicellular  plants,  178. 

Vacuoles,  24,  162,  170. 


INDEX. 


231 


Vascular  system,  54. 
Vas  deferens,  76. 
Veins,  126. 
Vejdovsky,  79,  81 
Venation,  129. 
Vessels,  116. 
Vinegar,  196. 
VlRCIIOW,  21. 
Vital  energy,  33. 
Vital  force,  33. 
Vitellus,  74,  78. 
Vortieella,  168,  172, 

White,  43. 


White  1)1  cod -cells,  64. 
Whirlpool,  2. 

WiNOGUADSKY,  197. 

Yeast,  178. 
Yeast,  bottom,  190. 
Yeast,  red,  191. 
Yeast,  top.  190. 
Yeast,  wild,  190. 

ZoOgloea,  194,  195. 
Zooids,  176. 
Zoology,  6,  7. 
Zoospores,  181. 
Zoothamnioa,  176. 


noPEXTT  uijuir 

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viii  'oa  3 


Henry  Holt  &-  Co.'s  Works  on  Science 

Price's  Fern  Collector's  Handbook  and  Herbarium.    With  72  Plates^.    4to 

Retail.  %2.2b. 

Randolph's  Laboratory  Directions  in  General  Biology.  By  Miss  Har- 
riet KANDOiiPH,  Instructor  in  Bryn  Mawr  College.     163  pp.       80c. 

Reigfhard  and  Jennings's  Anatomy  of  the  Cat-  By  Profs.  Jacob  Reto- 
HAKD  and  H.  S.  Jennings.  University  of  Michigan.    498pp.    8vo. 

4.00 

Sanford's  Elements  of  Physics.  By  Prof  F.  Sanford  of  Stanford  Univ. 
On  the  experimental  method.    426  pp.  1.20 

Schenck  and  Griiber's  Outline  of  Human  Physiology.  Translated  by  Dr. 
William  D.  Zoethout.  With  preface  by  Prof.  Jacques  Loeb, 
University  of  Chicago.    339  pp.    8vo.  1.75 

Scudder's  Butterflies.    By  S.  H.  Scudder.    322  pp.    12mo.  1.50 

Brief  Guide  to  the  Commoner  Butterflies.    With  21  plates,  contain 

ing  ^7  iUustrations.    206  pp.  ItetaiL  1.50 

The  Life  of  a  Butterfly.  A  Chapter  in  Natural  History  for  the  Gen- 
eral Reader     186  pp.    lOmo,  Retail,  1.00 

Torrey's  Elementary  Studies  in  Chemistry.    By  Joseph  Torrey.  Jr., 

Instructor  in  Harvard.     487  pp.     ]2mo.  1.25 

Underwood's  Our  Native  Ferns  and  their  Allies.  By  Prof.  Lucien  M. 
Underwood  of  Columbia.    Illustrated.    156  pp.  1.00 

Moulds,  Mildews,  and  Mushrooms.    Illustrated.   236  pp.  1.50. 

"Wheeler's  The  Elements  of  Animal  Embryology.  By  Prof.  William 
Norton  Wheeler,  University  of  Texas.    {In  jyreparation.) 

White's  Qualitative  Analysis.  By  Prof.  John  White.  Univ.  of 
Nebraska.  820  pp.  80c. 

"Williams's  Elements  of  Crystallography.  By  Geo  Huntington  Will- 
iams, late  Prolessor  in  Johns  Hopkins  University.   270  pp.  Revised- 

].25 

"Williams's  Geological  Biology-  An  Introduction  to  the  Geological 
History  of  Organisms.  By  Prof.  Henry  S.Williams  of  Yale.  8vo. 
395  pp.  2.80 

"Woodhull's  First  Course  in  Science.    By  Prof.  John  F.  Woodhull, 

Teachers'  College,  New  York  City. 

I.  Booh  of  Expei-iments.xiv-i- 79-pp.    8vo,    Paper.  50c. 

II.  Text-hook.    xv-r]33pp.     ]2mo.    Cloth.  65c; 

"Woodhull  and  VanArsdale's  Chemical  Experiments.  An  Elementary 
Manual  largely  devoted  to  the  chemistry  of  every-day  life.  Inter- 
leaved.   136  pp.  60c, 

Zimmermann's  Botanical  Microtechnique.  Translated  by  James  Ellis 
Humphrey.  S.  C.    xii  -h  296  pp.    8vo.  2.50 


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